1. The astronomical telescope. --The only parts essential to this telescope are two lenses,
Fig. 5.

[Illustration : A diagram showing how the image of an arrow is passed through two lenses before meeting the human eye. The parts of the diagram are labelled with letters.]
A D and E Y, fig. 5. A D is the object glass, and E Y is the eye glass. Let O B be a distant object, from which rays proceed nearly parallel


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to A D, the object lens. The rays passing through this lens will form an image of the object in its focus at I M, and as the rays cross each other it will be inverted. E Y, the eye glass, is placed exactly at its focal distance from this image, and must be of a much shorter focal distance than the object glass. The magnifying power in this telescope is in the proportion of the focal distance of the object glass to that of the eye glass. Suppose the object glass to be of 24 inches focal distance, and the eye glass 1 inch, the magnifying power is then as 1 to 24; in other words, it makes a distant object appear 24 times nearer, and 24 times larger in size than it is to the naked eye. If the eye glass had been 1 1/2 inch focal distance, the magnifying power would have been only 16 times. Through this telescope all objects appear inverted, like the image at I M. This telescope was much in use by astronomers in the seventeenth and eighteenth centuries. In order to obtain a considerable magnifying power, it was sometimes made more than a hundred feet long; and as it was found inconvenient to have a tube of this length, the telescope was used without one. A high pole was erected, the object glass was placed at the top of it, and capable of being turned in every


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direction by a cord which descended to the station occupied by the observer, who held the eye-piece at the height of his eye, resting it on a pedestal. But these long instruments are now entirely superseded by the invention of achromatic and reflecting telescopes.

A telescope of the kind described may be easily constructed, and at a small expense . Procure a double convex lens, about 36 inches focal distance, place it at the end of a tube,
Fig. 6.

[Illustration : A cross-section of an astronomical telescope. The parts of the telescope are labelled with letters.]
as at A B, fig. 6; at 37 inches distance from this glass fix another, C D, whose focal distance is 1 inch. This glass must be fixed in a separate short tube, E F, which should be made to slide backwards and forwards in the large tube, for the purpose of adjusting the focus of the telescope to the eye. Such a telescope will magnify objects in the proportion of 1 to 36, or 36 times, and will show the spots in the sun, the shadows of the mountains and cavities in the moon, Jupiter's satellites, the


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crescent of Venus, and other celestial phenomena. In constructing such a telescope, the following particulars require to be attended to:--1. The aperture, or opening which lets in the light at the object glass, should not exceed 1 inch in diameter, otherwise the image of the object will be somewhat confused. 2. There should be a hole or aperture in the focus of the eye glass rather less in diameter than the breadth of the eye glass, for the purpose of excluding the extraneous rays. Were a telescope of this kind to be constructed with an object glass 6 feet focal distance, the eye glass would require to be 1 1/2 inch focal distance, and the aperture of the object glass 1 1/2 inch diameter, and the magnifying power would be 45 times. Were the object glass 10 feet focus, the eye glass would require to be 1 9/10 inch focus, and the aperture 1 7/10 inch, and the magnifying power would be 63 times. Were the object glass 20 feet focus, the eye glass would be 2 1/2 inches focus, the aperture 2 4/10, and the magnifying power would be 89 times. It is found, that in order to magnify twice as much as before, with the same light and distinctness, the telescope must be lengthened 4 times; to magnify three times as much, 9 times; four


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times as much, 16 times; that is, suppose a telescope of 3 feet to magnify 36 times, in order to procure a power four times as great, or 144 times, we must extend the telescope to the length of 48 feet, or 16 times the length of the other.

2. The common refracting telescope for land objects. --The telescope just described, in consequence of its showing every object in an inverted position, is not fitted for land objects. In order to adapt it for terrestrial objects two additional eye glasses are required. In fig. 7, let O B represent a distant object, L N the object
Fig. 7.

[Illustration : A diagram showing light rays projecting the image of an object passing through four different lenses. The parts of the diagram are labelled with letters.]
glass which forms the image I M in an inverted position; let E E represent another glass placed at its focal distance from this image, as in the astronomical telescope, and F F a second glass, placed at twice its focal distance from E E. By this glass ( F F ) a second image is formed at i m, contrary to the first image I M, and, consequently,


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erect. This last image is viewed by the third lens, G G, in the same manner as the first image I M would be viewed by E E. All these eye glasses are understood to be of the same focal distance, and are placed at double their focal distance from each other. Such a telescope represents land objects in their natural positions. But this arrangement of glasses is different from the eye-piece now adapted to achromatic telescopes, of which we shall afterwards give a short description.

3. The Galilean telescope. --This telescope consists of two glasses, a convex glass, C, next the object, and a concave, K, next the eye, fig. 8.
Fig. 8.

[Illustration : A diagram showing rays of light passing through a double convex and a double concave lens. The parts of the diagram are labelled with letters.]
Let I D represent rays proceeding from an object; these rays would converge to their foci, and form an inverted image at F, if they were not intercepted by the concave lens K. But


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this lens being a double concave, occasions the rays to diverge more than before, so that the rays D I emitted from the object, instead of converging to F, are made to proceed parallel to G H, and it is parallel rays that produce distinct vision. The concave lens, K, is placed as much within the focal distance of the convex, C, as is equal to its own focal distance, and the magnifying power, as formerly, is in proportion to the focal distance of the object glass to that of the eye glass. Thus, suppose the focal distance of the object glass to be 12 inches, and that of the concave eye glass 1 inch, the concave must be placed at 11 inches from the object glass, and the magnifying power will be as 1 to 12, or 12 times. This was the kind of telescope constructed by Galileo, and with which he made his astronomical observations. Objects are seen through this telescope in their natural position, and very distinct, even more so than in other telescopes; but the field of view is so very small that its use is almost exclusively confined to the common opera glasses. A good telescope, however, can be made on this principle capable of showing much to interest the observer. If we take a convex lens, 40 inches in focal distance, we may apply to it a concave lens of only


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1 inch focal distance, at the distance of 39 inches from the object glass, and the magnifying power will be 40 times, which will show us Jupiter's satellites, the crescent of Venus, and Saturn's ring. There is some difficulty in finding an object with this telescope, and only a part of the sun and moon can be seen through it at one time.

Manner in which achromatic telescopes are fitted up for astronomical observations. -- T, in fig. 11, is the body of the telescope mounted on a tripod pedestal of brass. A is the eye-piece, which screws off to admit other eye-pieces to be applied. E is the long eye-piece for terrestrial objects. D is an astronomical eye-piece. B consists of two or three sliding tubes of brass, for rendering the instrument steady; and a is a brass knob, which moves a piece of rack work for adjusting the focus. a b the finder .

This telescope is now generally used. If flint glass of a large diameter, and without blemishes, could easily be procured, it would


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Fig. 11.

[Illustration : An illustration of an achromatic telescope set up on a stand, with two specialized eyepieces illustrated next to it. The parts of the telescope and the extra eyepieces are labelled with letters.]
soon supersede almost all other telescopes. But hitherto it has been very difficult, in Britain, to procure large discs of flint glass, of a good quality, unless at a great expense. We have been chiefly indebted for the largest discs of flint glass, and for the best achromatic telescopes


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we possess, to the French and Germans, who charge high prices for such articles. But we have now some prospect that glass for achromatic purposes may be procured in greater abundance, and at a more moderate cost, than formerly. When an achromatic object glass is prepared, and fitted in a tube, we apply magnifying powers to it in the same way as to the common refractor; only we can apply to it eye glasses of a much shorter focal distance--such as half or quarter of an inch--than we can apply to an object glass with a single lens. But as the eye-piece for achromatic telescopes is different from that formerly described, it may be expedient here to give a short description of it.

Astronomical eye-pieces. --The combination of lenses now most frequently used for astronomical observation is that which is denominated the Huygenian eye-piece, which is a great improvement on the eye-piece with a single lens, as it produces a more distinct and enlarged field of
Fig. 13.

[Illustration : A diagram showing how the lenses of a Huygenian eyepiece, both plano-convex, enlarge the image of an object. The parts of the diagram are labelled with letters.]
view. Let A B, fig. 13, be a compounded pencil of white light proceeding from the object glass, B F a plano-field glass, with its plain side next the eye glass E. The red rays of the pencil, A B, after refraction, would cross the axis in R and the violet rays in V, but meeting the eye


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glass E, the red rays will be refracted to O, and the violet nearly in the same direction, when they will cross each other about the point O in the axis, and unite, so as to form an eye-piece almost without colour. The distance of the two glasses, F E, to produce this correction, must be equal to half the sum of their focal distances. Suppose the focal distance of the field lens F to be 3 inches, and that of E, the lens next the eye, 1 inch, the two lenses should be placed exactly at the distance of 2 inches; in other words, the glass next the eye should be placed as much within the focus of the field glass as is equal to its own focal distance. The focal length of a single lens that has the same magnifying power as this compound eye glass, is equal to half the focal length of the field glass. In the present case, the focal length of a single lens which would produce the same magnifying power would be 1 1/2 inch. The proportion of the focal lengths of the two lenses to each other should be as 3 to 1, that is, if the field be 1 1/2 inch, the eye glass should be 1/2 inch. An astronomical eye-piece may also be formed with two plano-convex glasses, placed with their convex sides towards each other, and at a very small distance from


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each other, namely, somewhat less than the focal distance of the lens next the eye. If, for example, one of the lenses were 1 inch focal length, and that next the eye 3/4 of an inch, they might be placed at somewhat less than 1/2 inch asunder.

Diagonal eye-pieces. --When a celestial object is at a high altitude, the observer is obliged to put his head in a very inconvenient position, and to direct his eye nearly upwards, in which position a steady view of the object can scarcely be obtained. The diagonal eye-piece has been invented to remedy this inconvenience. This is effected by placing a flat piece of polished
Fig. 14.

[Illustration : A diagram of the interior of a diagonal eyepiece. The parts of the eyepiece are labelled with letters.]
speculum metal, D C, fig. 14, at an angle of 45° to the axis of the tube, A B. This part slides


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into the tube of the telescope, E F ; the tube containing the lenses stands at right angles to the position of the telescope, and slides into an exterior tube, a n, i o, and the eye is applied at G. This construction of the diagonal eye-piece may be used either with the Huygenian eye-piece before described, or with that which is formed of two plano-convex lenses, with their convex sides towards each other. They may be of any magnifying powers generally applied to the telescope, and may be changed at pleasure. The rays proceeding from the object glass and falling upon the plain speculum, D C, are reflected in a perpendicular direction to the eye-piece, E F, and enter the eye at G. When this eye-piece is directed to celestial objects at a high elevation, the observer may either sit or stand, and look down upon the object with perfect ease. When it is directed to terrestrial objects, the spectator likewise looks down upon them, but they present a novel aspect, and when the telescope is moved from one side to another, they have the appearance of a moving panorama. When the eye-piece is turned round a quarter of a circle towards the right, those objects which are in the south will appear as if they were in the east; when it is turned a


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quarter of a circle towards the left, they will appear as if they were in the west; and when it is turned half round from its first position, they will appear as if they were above us, suspended in the air.

Fig. 15.

[Illustration : An illustration of a telescope with a diagonal eyepiece set up to observe sunspots. The telescope is set in front of a window, which has been completely shaded, except for a small hole through which the telescope is set to view. The image of the sunspots is reflected through the diagonal eyepiece and projected onto the ceiling of the room.]

A telescope furnished with a diagonal eye-piece is the most convenient for exhibiting the spots in the sun. The window shutters should be all closed, having a small opening sufficient


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to admit the solar rays, and when the telescope is properly adjusted, a large and beautiful image of the sun, with all the spots which then happen to diversify its surface, is thrown upon the ceiling of the room, which must be of a white colour. In this way we may measure with a scale the diameter of the solar image, and also the diameter of a large spot, and by comparison we may determine the exact diametrical proportion of the latter.

1. The Newtonian reflector. --This instrument is represented in fig. 16, where B A E F is the tube, in which is placed the concave speculum B E, which reflects the parallel rays O c, P d, to a plane speculum G, which is placed at half a right angle to the axis of the tube, as much


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nearer the speculum than its focus as the centre of the small mirror is distant from the tube, that is, the distance G h of the small speculum from the focus of the great one should be equal to G H, half the diameter of the tube. This small speculum should be of an oval form, the length of which should be to the breadth as 7 to 5. It is supported by an arm fixed to the side of the tube. The rays, O c, P d, which
Fig. 16.

[Illustration : A diagram of the Newtonian reflector. The parts of the diagram are labelled with letters.]
form the image of the object by reflection, instead of proceeding to form it at h, are intercepted by the plane speculum at G, and reflected upwards to an aperture in the side of the tube A B, where the image is formed, and magnified by a convex lens of a short focal distance, to which the eye is applied, looking downward on the object, which appears inverted; or, the eye glass, instead of being on the upper part of the tube, may be placed on one of the sides, and for viewing land objects a terrestrial eye-piece may be applied. The magnifying power of this telescope is in the proportion of the


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focal distance of the speculum to that of the eye glass. Thus, if the focal length of the speculum be 60 inches, and that of the eye glass a quarter of an inch, the magnifying power will be 240 times. This is the form of the reflecting telescope which was most generally used by the late sir W. Herschel. The following table exhibits a statement of the diameters of the specula of Newtonian reflectors, the focal distances of their eye glasses, and their magnifying powers:--

Focal distance of concave speculum.	Aperture of concave metal.	Focal distance of single eye glass.	Magnifying power.
Feet.	In. Dec.	In. Dec.
1	1 · 44	0 · 99 or 1/5 [unclear]	60
2	2 · 45	0 · 236	102
3	3 · 31	0 · 261	138
4	4 · 10	0 · 281	171
5	4 · 85	0 ·297	202
6	5 · 57	0 · 311	232
7	6 · 24	0 · 323	260
8	6 · 89	0 · 334	287
9	7 · 54	0 · 344	314
10	8 · 16	0 · 353	340
12	9 · 36	0 · 367	390
15	11 · 04	0 · 391	460

It will generally be found, that the power produced by multiplying the diameter of the


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speculum by thirty or forty, is most satisfactory for planetary observations.

2. The Gregorian reflector. --This form of the reflecting telescope is constructed in the following manner:-- T T T T is the great tube, open at the end next the object, in which the large concave speculum D U V F is placed, whose principal focus is at m, and in its middle is a round hole P, opposite to which is placed the small mirror L, concave towards the great speculum, and so fixed on a strong wire M, that it may be moved further from or nearer to the great mirror, by means of a long screw on the outside of the tube. The rays proceeding from the object A B, and falling on the speculum D F will be reflected to its focus m, where an inverted image of the object will be formed.

This image is formed at a little more than the focal distance of the small speculum from its surface, and the small mirror acting upon it, this first image is reflected through the glass R to a b, where a second image is formed erect, and larger than the first in the proportion of b K to K g. This image is again magnified by the eye glass S, to which the eye is applied.


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Fig 17.

[Illustration : A diagram showing the inside of a Gregorian reflector telescope, and how it transmits an image to the eye. The parts of the diagram are labelled with letters.]



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The rays from the glass S pass through the small hole e, which should seldom be more than 1/25 of an inch in diameter. To find the magnifying power of this telescope, multiply the focal distance of the great speculum by the distance of the small speculum from the image next the eye; and multiply the focal distance of the small speculum by the focal distance of the eye glass; then divide the product of the first multiplication by the product of the last, and the quotient will be the magnifying power.

3. Cassagrainian reflector. --This kind of reflector is constructed in the same way as the Gregorian, with this difference, that a small convex speculum is substituted in the room of the small concave L, fig. 17. This convex mirror is placed as much within the focus of the great speculum as is equal to its own focal distance. Thus, if the focal length of the large speculum be 24 inches, and that of the small convex 2 1/2 inches, they are placed at 21 1/2 inches from each other, and instead of two, there is only one image formed, namely, that in the focus of the eye glass. The length of this telescope is less than that of a Gregorian by twice the focal length of the small mirror.


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From the experiments of Short, Ramsden, captain Kater, and others, it appears that there is more light in this telescope than in the Gregorian; and that it is, on the whole, superior in its performance, but it represents the object in an inverted position. *

1. Large achromatic telescopes. --(1.) The great Cambridge telescope, in Massachusetts, United States. --This instrument was procured from Munich, in Germany, at a vast expense,

*Besides the telescopes described above, there are plans of others by which distant objects may be viewed. 1. A telescope may be made of a single lens of a long focal distance. The writer has a lens 26 feet focal distance and 11 1/2 [unclear] diameter, which, without any other glass, produces a magnifying power of nearly 30 times, and by which he has read the hour of the day on a public clock two miles distant. The observer stands at a distance of about 25 feet from the lens, his eye serving as the eye glass, on the principle of the Galilean telescope. 2. Theaėrial telescope, constructed by the writer, which has only one speculum, and in looking through which the observer sits with his back to the object. This telescope has no tube, but only a short socket to hold the speculum. An arm at one side extends the length of the focal distance of the speculum, at the end of which is the eye-piece. 3. The Newtonian telescope may also be fitted up without a tube, which saves considerable expense. The writer has fitted up one on this plan, which performs admirably. It may be changed at pleasure into the aėrial telescope.

2. Large reflecting telescopes. --Mr. James Short, of Edinburgh, was the first who made reflecting telescopes on a large scale. In 1743, he constructed one for lord Spencer, of 12 feet focal length, for which he received 600 guineas; and, in 1752, he finished a still larger one for the king of Spain, for which he received £1,200. This was considered the noblest instrument of its kind till Herschel constructed his large reflectors. About the year 1780, the late sir W. Herschel constructed a Newtonian reflector, 20 feet long, with which he explored the Milky Way, and other objects in the sidereal heavens. In 1789, he finished his large 40 feet telescope, which at that time was unrivalled. Its speculum


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was 4 feet diameter, and it had neither a concave nor plane small speculum, but the observer sat with his back to the object, and looked down upon the great speculum. This telescope was dismantled a few years ago. A large telescope, 20 feet in length, has lately been constructed by Mr. Larsels, of Liverpool, with which he has discovered several small planets. But the largest reflectors now in existence are those which have been lately erected by the earl of Rosse. One of these, in the lawn before his lordship's mansion, is 27 feet long, and its speculum 3 feet diameter. Another, called the "monster telescope," is 56 feet long, and its speculum 6 feet in diameter, and weighs above 3 tons. This is the largest telescope in the world, and its erection cost his lordship £12,000. It is of the Newtonian form.

1. Achromatic telescopes. --"The improved 2 1/2 feet achromatic, on a brass stand, mahogany tube, with three eye-pieces, two magnifying about 40 or 50 times, and the other about 75 for astronomical purposes, in mahogany case, £10. 10 s. ; ditto, with brass tube, £11. 11 s. ; ditto, with vertical and horizontal rackwork motions, £15. 15 s. " This telescope, if the object glass be good, will bear a power of 90 or 100 times for celestial objects. Its object glass is 2 1/4 inches diameter. "The 3 1/2 feet achromatic, plain mahogany tube, 2 3/4 aperture, £18. 18 s. ; ditto, brass tube, £21; ditto, all in brass, with rackwork motions, £26. 5 s. ; ditto, the object glass of 3 1/4 inches aperture, and the rack motions on an improved principle, from £37. 16 s. to £42." The magnifying powers of these telescopes are from 130 to 180 or 200 times. This is the telescope which we would recommend to astronomical observers. It will show all the common phenomena of the solar system. The 5 feet achromatic is also frequently made; diameter of the object glass 3 8/10 inches; powers, 65, 110,


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190, and 250. Its general price is above 100 guineas. Achromatic object glasses for such telescopes may sometimes be purchased separately, at such prices as the following:--Focal length 30 inches, diameter 2 1/4 inches, from 2 to 2 1/2 guineas; focus 42 inches, diameter 2 3/4 inches, from 5 to 8 guineas; focal length 42 inches, diameter 3 1/4 inches, from 12 to 20 guineas. Eye-pieces may be procured from 10 s. 6 d. to 14 s.

2. Reflecting telescopes. --The reflectors commonly made and sold in London are the following:--

A 4 feet 7 inch aperture, Gregorian reflector, with the vertical motions upon a new principle, with apparatus to render the tube more steady for observation, according to the additional apparatus of small specula, eye-pieces, micrometers, etc., from £80 to £120; 3 feet long, mounted on a brass stand, £23. 2 s. ; ditto, with rackwork motions, improved mountings and metals, £39. 18 s. ; 2 feet long, without rackwork, and with 4 magnifying powers, £15. 15 s. ; ditto, improved, with rackwork motions, £22. 1 s. ; 18 inch, on a plain stand, £9. 9 s. ; 12 inch ditto, £6. 6 s.

Discoveries in the solar system.

Discoveries made by the telescope in the sidereal heavens.

Of late years, the telescope has been more particularly directed to the starry heavens than in former times, and many wonderful discoveries have been made, of which it is impossible to give more than a meagre outline.

Such is a very brief sketch of the discoveries in the heavens which have been made by the aid of the telescope. Before its invention, only seven of the heavenly bodies were known to belong to our system, namely, the Sun and Moon, Mercury, Venus, Mars, Jupiter, and Saturn. Since its invention, fourteen primary and twenty secondary planets have been discovered, together with millions of fixed stars, the existence of which had not been previously imagined. To its assistance we also owe all that is known of variable stars, double stars, clusters, nebulę, the Milky Way, and of the true magnitudes of the bodies which compose the solar system. *

1. How impressively do the discoveries of the telescope illustrate the almighty power of God! Let the reader reflect for a moment upon the view which this admirable instrument presents of the extent of creation. There was a time, when the conception of neither poet, philosopher, nor divine, soared beyond the sphere of unaided human vision; and when, looking upon the earth as the largest body in the universe, and the sun, moon, and stars, as mere appendages to enlighten and adorn it, they imagined all the Creator's works to be confined within these narrow bounds. But the telescope has opened to us a field to which no limits can be assigned. Within our own planetary system, it has discovered to us bodies more than a thousand times larger than the globe we inhabit; and in the distant starry regions, beyond our nearer circle, it has shown to us myriads of suns,


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equal in magnitude to the one which rules our day, and each the centre of other worlds, all of which are probably teeming with happy dwellers. Nor is this all; for as one bright scene of splendour rises above another, in almost boundless perspective, who can doubt that others, yet more wonderful and sublime, lie beyond the range of the most powerful telescope? When it has taken us to the farthest line of observation it can reach, who will say that "the wonders of the Almighty are at an end, because we can no longer trace his footsteps?--that his omnipotence is exhausted, because human heart can no longer follow him?"

Scarcely less calculated to raise our conceptions of the almighty power of God, is the magnitude of the objects which the telescope brings under our notice. The planet Jupiter is computed to be more than thirteen hundred times larger than the earth. The sun is five hundred times larger than all the planets and comets would be, were it possible to unite them in one vast globe. The star alpha Lyrę is reckoned to be three million two hundred and seventy-five thousand miles in diameter, and more than fifty-four thousand times larger than the sun. Omnipotence


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is implied in the existence of an atom as well as of a world, but we realize that attribute more vividly in the latter than in the former. We look at our own globe, with all its mighty oceans and continents, containing more than two hundred and sixty thousand millions of cubical miles of solid matter, and we are astonished at the power of the Creator; but how is that astonishment increased, when we learn from the discoveries of the telescope, that, instead of being distinguished from other worlds, it is one of the least of them--a mere atom in creation; and that, in comparison with other systems, the one in which this atom is found is but a mere shred, which, though scattered into nothing, would leave the universe of God one entire scene of greatness and of majesty!

To these considerations may be added the velocity with which these stupendous bodies move in their courses. The planet Saturn, with its rings and moons, moves in its circuit round the sun at the rate of twenty-two thousand miles an hour. The planet Venus performs its revolution at the rate of eighty thousand miles an hour, and the planet Mercury flies through


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its orbit at the extraordinary speed of one hundred and nine thousand miles an hour. The motion of some of the fixed stars, a million times larger than the earth, has been calculated to be one hundred and seventy thousand miles an hour, and the velocity of some of the comets is known to be no less than eight hundred thousand miles an hour, or more than thirteen thousand miles a minute. When we reflect, too, on the size and number of the heavenly bodies, on the probability that they all move with a rapidity which, if it be not impossible to calculate, is most difficult to conceive, and that there is an order and harmony in their motions which prove them to be controlled and regulated by a will as omnipotent as that which called them into existence, we cannot refrain from exclaiming with the psalmist, when he thought upon the glories of the firmament, "Great is our Lord, and of great power," Psa. cxlvii. 5.

2. How striking and beautiful is the coincidence between these discoveries of the telescope and the representations of Scripture! It is impossible to determine whether the wonders which modern astronomy has brought to light


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were revealed to the sacred writers. But how remarkably consistent is their language with all the wonders which the telescope has revealed! Where would the devout astronomer, laying down the instrument with which he has explored the heavens, find the most suitable expression for those feelings which have been awakened by what he has there seen of the glory and power of the eternal Creator? Where would he find language more elevated and appropriate than in the sacred volume? And in this concurrence between science and Scripture, have we not an additional confirmation of the fact, that the God of nature is the God of the Bible?

3. All subjects, as they are exhibited in the Bible, have reference to the great work of redemption. The cross of Christ is a centre from whence the Christian will survey the universe. The hill of Calvary will be his observatory. Oh! how unutterable will be the emotions with which he will "consider" the heavens, when he remembers that their almighty Maker is his Redeemer! "All things were made by him; and without him was not anything made that was made," John i. 3.


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There is a rapture felt in gazing on the starry sky, which can be only known to him in whose heart the Saviour is enthroned. He will see all the glories of creation reflected on the mediatorial work of that Saviour, and every discovery which expands his views of the former will exalt his conceptions of the latter. "Those worlds of light," he will say, "those countless suns and systems which the telescope brings within my sight, and in the contemplation of which, either individually or collectively, I am lost in astonishment and filled with awe, are all the workmanship of my Redeemer. And is it so? Then how transcendent must be his glory! Do I go to Bethlehem, the place of his incarnation, and see him,
"On whom the vast universe hung," as an infant cradled in a manger? What an infinite depth of condescension! Do I visit the place of his crucifixion, and see him bearing "our sins in his own body on the tree?" 1 Pet. ii. 24. How inestimable must be the value of the sacrifice! Does my faith follow him from the scene of his abasement to that of his exaltation? Do I behold him as Mediator, invested with absolute and unlimited sovereignty? How is my confidence strengthened! Surely "he is


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able also to save them to the uttermost that come unto God by him," Heb. vii. 25.

Nor let it be imagined that the magnitude of creation affords any reasonable objection to the scheme of redemption as revealed in the Bible. The objection has been made, but its whole force is met in the one simple consideration, "that God, in addition to the bare faculty of dwelling on a multiplicity of objects at one and the same time, has this faculty in such wonderful perfection, that he can attend as fully, and provide as richly, and manifest all his attributes as illustriously, on every one of these objects, as if the rest had no existence, and no place whatever in his government or in his thoughts." * And in illustration and proof of this position, let it be remembered, that soon after the invention of the telescope, which put infidelity in possession of the objection, another instrument was found, "which laid open a scene no less wonderful, and rewarded the inquisitive spirit of man with a discovery which serves to neutralize the whole of this argument. This was the microscope. The one," said that great

*"Discourses on the Christian Revelation, viewed in connexion with modern Astronomy," by Rev. Dr. Chalmers.

"Now mark how all this may be made to meet the argument of our infidel astronomers. By the telescope they have discovered that no magnitude, however vast, is beyond the grasp of the Divinity. But by the microscope we have also discovered, that no minuteness, however shrunk from the notice of human eye, is beneath the condescension of His regard. Every addition to the powers of the one instrument, extends the limit of his visible dominions. But, by every addition to the powers of the other instrument, we see each part of them more crowded than before with the wonders of his unwearying hand. The one is constantly widening the circle of his territory. The other is as constantly filling up its separate portions with all that is rich, and various, and exquisite. In a word, by the one I am told that the Almighty is now at work in regions more


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distant than geometry has ever measured, and among worlds more manifold than numbers have ever reached. But, by the other, I am also told, that, with a mind to comprehend the whole in the vast compass of its generality, He has also a mind to concentrate a close and separate attention on each and on all of its particulars; and that the same God who sends forth an upholding influence among all the orbs and movements of astronomy, can fill the recesses of every single atom with the intimacy of his presence, and travel, in all the greatness of his unimpaired attributes, upon every one spot and corner of the universe he has formed." *

For the introduction of a passage so long, and so well-known, the reader will find an apology in its great beauty and appositeness. It may also be added, that it would be impossible to supply a more appropriate preface to the subject of the following pages. It is a link of exquisite workmanship, connecting the two parts of this little volume.

1. The single microscope. --This simplest of all microscopes is nothing more than a convex lens, whose focal distance is extremely short.
Fig. 18.

[Illustration : A diagram of a person looking through a double convex lens at an arrow, showing how the lens magnifies the image. The parts of the diagram are labelled with letters.]
Let A B, fig. 18, be a double convex lens, F E the object at its focus C, G the eye very near


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the lens A B ; the rays coming from the object will, after their refraction, fall parallel upon the eye, and, consequently, make distinct vision. Therefore, a minute object E F, seen distinctly through a small glass lens A B, by the eye put close to it, appears so much greater than it would to the naked eye placed at the distance H D, as this distance is greater than H C. To illustrate this, let us suppose the focal distance of the glass A B to be half an inch, and the distance H D eight inches, the usual distance at which we view minute objects, then the object may be said to be magnified as much as eight inches exceed the small space H C, or the focal distance of the lens A B, that is, in the proportion of 16 to 1, or 16 times. If the focal distance of the lens were 1/6th of an inch, the magnifying power would be 40 times; if 1/10th of an inch, 80 times; and if 1/20th of an inch, the diameter of any object would be magnified 160 times, which is found by dividing 8 inches by the focal length of the lens, 1/20 ÷ 8 = 160. The surface of the object will, of course, be found by multiplying the diameter into itself, which produces 25,600 times, and the solidity or bulk would be magnified 4,096,000 times, that is, the surface multiplied


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by the diameter. A single microscope may be represented by fig. 19, where V is the lens fixed into a socket with a handle, A E a small object placed at its focal distance from the lens, and a e the magnified picture of the object.

Fig. 19.

[Illustration : A diagram showing a person looking through a lens at an insect; the image of the insect is shown magnified to the left of the actual insect. The parts of the diagram are labelled with letters.]

The performance of the single microscope depends, in a great measure, on the clearness and purity of the glass of which it is made, and on the accuracy with which it is polished, so as to keep it of a true spherical figure. When completed--that is, when ground and polished--it should be as thin as it can possibly be rendered with a sufficient aperture. When a lens is thick, approaching to the figure of a globe, it is not so transparent as when thin, and the field of view at the edges is partly distorted. And it must be of a sufficient diameter or aperture, that the eye may take in a moderate


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field of view, and that there may be as little deficiency of light as possible. Lenses have been made whose focal length did not exceed 1/40th, 1/50th, or 1/60th of an inch; but such high powers are difficult to be used. Sir D. Brewster has remarked, that "we cannot expect any essential improvement in the single microscope, unless from the discovery of some transparent substance, which, like the diamond, combines a high refractive power with a low power of dispersion." In correspondence with this suggestion, the diamond has been of late years formed into lenses by Mr. Pritchard, of London, who commenced the undertaking in July, 1824. The first diamond lens was completed at the end of that year, and notwithstanding the difficulty of working this substance into a perfect figure, he ultimately overcame it, and finished the first diamond microscope in 1826. The focal distance of this magnifier, which was double convex, is about 1/30th of an inch. The principal advantages of employing diamonds in the formation of microscopes arise from the naturally high refracting power they possess, by which we can obtain lenses of any degree of magnifying power, with comparatively shallow curves. The indistinctness


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occasioned by the figure of the lens is thus greatly diminished, and the dispersion of colour in the substance being as low as that of water, renders the lens nearly achromatic. Mr. Pritchard has also formed lenses of sapphire and other precious stones, but they are not preferable to the diamond. The following table exhibits the magnifying powers of Mr. Pritchard's sapphire microscopes:--

Parts of an inch.	Magnifying Power. Linear.	Magnifying Power. Superficial.
1/10	80	6,400
1/15	120	14,400
1/20	160	25,600
1/25	200	40,000
1/30	240	57,600
1/40	320	102,400
1/50	400	160,000
1/70	560	313,600
1/80	640	409,600
1/100	800	640,000

In mounting the diamond and sapphire lenses there are advantages which glass lenses do not possess. Their extreme hardness enables them to be burnished with brass settings, which is very difficult with those of glass. This facility of mounting renders them more extensively


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useful in experimental researches, from their capability of being applied in every possible way with regard to the object, the light, or the eye. But it is evident that such lenses, both from the difficulty of grinding and polishing, and from the costliness of the material, must be very expensive.

There are various simple methods of procuring small lenses for microscopes, some of which may be here stated. Take a small slip of window glass, about 1/10th of an inch broad; melt it in the flame of a lamp, then draw it out into fine threads, then hold one of these threads with its extremity in or near the flame, till it runs into a globule. The globule may then be cut off and placed above a small aperture, so that none of the rays which it transmits pass through the part where it is joined to the thread of glass. Lenses composed of fluids have also been made, which are frequently useful where better microscopes are not at hand. Take up a drop of water on the point of a pin, and place it in a small hole in a thin piece of brass, about 1/20th of an inch in diameter. The hole should be in the middle of a small spherical cavity, about 1/7th of an inch in diameter, and


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a little more than half the thickness of the brass, which should not exceed 1/16th of an inch in thickness. On the opposite side of the brass should be another spherical cavity, half as broad as the former, and so deep as to reduce the circumference of the small hole to a sharp edge. The water being placed in these cavities will form a double convex lens with unequal convexities, which will produce a pretty high magnifying power. A better substitute for water is a drop of very pure and viscid turpentine varnish, which may be taken up on the point of a piece of wood and dropped upon a piece of thin and well-polished glass. Sir D. Brewster describes the following as the best method of constructing fluid microscopes:--Take Canada balsam, castor oil, or pure turpentine varnish, and drop either of them on a piece of glass, the surfaces of which are parallel, when a plano-convex lens will be formed. Their power may be varied by the quantity of the fluid employed, or by allowing the plate of glass to be horizontal with the drop above or beneath it: if the plate be uppermost, the gravity of the fluid will make it more convex; if the drop be above the plate, the lens will be flattened. When turpentine is used, it


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soon becomes indurated, and, if kept from dust, very durable. Sir David informs us, that he has made both the object and eye lenses of compound microscopes in this manner, which performed extremely well, and lasted a considerable time. * A single reflecting microscope may be formed by a concave speculum, having the object placed on its axis, and nearer to the surface of the reflector than the focus, when an enlarged view of the object will be seen on looking into the mirror. This instrument may be employed to enable a person to view his own eye, and will show a magnified representation of the ball, the pupil, the iris, and the ramifications of the blood vessels. On the same principle, if the reflector be large, for example, six inches in diameter, the whole head and face may be seen magnified three or four times in length and breadth, and above ten times in surface. There is a species of lens, sometimes called the Coddington lens, formed of a piece of glass nearly half, an inch in thickness. The upper and lower surfaces are convex. The sides are hollowed out, giving the lens somewhat the shape of an hour-glass, and reducing the stem to a very small size. These lenses are

*Treatise on New Philosophical Instruments, pp. 414, 415.

There are various modes of fitting up single


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microscopes, some of which are complex and expensive. The following plan may be recommended as both simple and convenient. Fig. 20 (p. 107) represents the mounting of a single microscope. K represents the box containing the whole apparatus. On its top there is a hollow screw, for receiving the screw that is in the bottom of the pillar A. D is a brass pin that fits into the pillar. On the top of this pin is a hollow socket to receive the arm that carries the magnifiers; the pin is to be moved up or down to adjust the lenses to their proper distance from the object. E the bar which carries the magnifying lens, which fits into the socket X. This arm may be moved backwards and forwards in the socket X, and sideways by the pin D, so that the magnifier, which is screwed into the ring at the end E, may be easily made to traverse over any part of the object that lies on the stage or plate B. FF is a polished silver speculum, with a convex lens placed at its centre, which is perforated for this purpose. The silver speculum screws into the arm E, as at F. H the semicircle which supports the mirror I. B the stage or the plane on which the objects are to be placed. L a pair of nippers which are fixed on the stage by the pin R. N an


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ivory slider which occasionally screws to the point of the nippers. The silver speculum is intended to throw light on the upper surface of an opaque object, but when transparent objects are viewed, there are other lenses which may be used without the speculum. Additional apparatus is connected with this and other microscopes, which it is unnecessary here to describe.

2. The compund microscope. --When a microscope consists of two or more lenses or specula, it is called a compound microscope. In this microscope the image is contemplated instead of the object; one of the lenses of which it is composed forms an image or picture of the object, as in the telescope, and this image, in a magnified state, is viewed by an eye glass, which produces an additional magnifying power. Let L N, fig. 21, represent a double convex lens, and O B a small object, so applied that the pencils of rays which emerge from it and pass through the lens may converge to their respective foci, and form an inverted image at I M. This image will be so much larger than the object, in proportion as its distance exceeds that of the object from the lens. For example, if the


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distance of the lens L N from the object O B be half an inch, and the distance L M, where the image is formed, be 7 inches, the image will be 14 times larger than the object; and if it be viewed through the lens F G, which suppose to be
Fig.21.

[Illustration : A diagram showing how the use of two double convex lenses more greatly magnifies an object, but renders only a small field of view. The parts of the diagram are labelled with letters.]

Fig. 22.

[Illustration : A diagram showing how the addition of a large double convex lens, for a total of three lenses, refracts the image to make it fully visible by the eye--in other words, it widens the field of view. The parts of the diagram are labelled with letters.]
1 inch, it will again be magnified 8 times, on the principle of the single microscope, and the whole magnifying power will be 8×14=112


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times. But the pencil of rays emitted from B in the object, and made to converge by the lens to M, proceeds afterwards diverging towards H, and, therefore, never arrives at the lens F G, nor enters the eye at E. Only the rays that proceed from the points o and b will be received on the lens F G, and by it be carried to the eye; the parts of the image i and m will be visible, but those situated towards I and M will not be seen. This quantity ( i m ) of the image I M is called the field of view, which is comparatively small when only a single glass is used. In order to enlarge the field of view, it is requisite that a broad lens, D E, fig. 22, be interposed at a small distance from the focal image; for by that means the pencil B M, which would otherwise have proceeded towards H, is refracted to the eye, as shown in the figure. In the same manner, the other extreme of the image is seen at Q, and the intermediate points are also rendered visible. On these considerations it is that compound microscopes are usually made to consist of an object lens, L N, by which the image is formed and enlarged, an amplifying lens, D E, by which the field of view is enlarged, and an eye glass, F C, by which the eye is allowed to approach very near, and to view the image


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Fig. 23.

[Illustration : An illustration of a compound microscope. The parts of the microscope are labelled with letters.]

Fig. 24.

[Illustration : An illustration of the nippers used to hold small objects under the microscope.]

Fig. 25.

[Illustration : An illustration of a small glass tube, containing a fish.]

Fig. 26.

[Illustration : An illustration of a microscope slide.]

Fig. 27.

[Illustration : An illustration of a small cone, used to diffuse light from the microscope when necessary.]

Fig. 28.

[Illustration : An illustration of a small pair of forceps.]

Fig. 29.

[Illustration : An illustration of a small piece of glass, used to contain small live animals for inspection under the microscope.]



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under a great angle of apparent magnitude. For similar reasons three, and sometimes four eye glasses are substituted in place of the amplifying lens.

Having briefly described the theory and principle of this microscope, we shall now give a description of the finished instrument, and the way in which it is used. The large figure, fig. 23, represents the body of the microscope ready for use, which, including the pedestal, is from 12 to 15 inches in height. In this figure it is represented as consisting of three tubes. In the large tube, A, the smaller one, B, slides up and down. At the upper part, a, one or two eye glasses are contained, and at some distance beneath them the amplifying lens is placed. At the lower part, b, the object glass is placed, and the small tube which contains it is connected with the tube B, by which it is made to slide up or down, to adjust the focus to the eye. Below the object glass is c, a kind of spring to receive the sliders, which may be occasionally taken out, and a plane glass laid across the opening in the stage on which any small object may be laid. D is the pedestal on which the instrument stands. C is a glass concave


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mirror, which turns in all directions, to reflect the light from a candle, or from a window, through the hole c. Fig. 24 represents nippers for holding insects, or other small objects. Fig. 25 is a small glass tube, capable of containing a live fish, when observing the circulation of the blood in its tail. Fig. 26 represents one of the sliders for holding objects, which are placed between two pieces of talc, or two thin slips of glass. Fig. 27 is a hollow cone, to be placed occasionally under the stage d, to diminish the quantity of light. Fig. 28 is a pair of brass forceps, to take up a minute object. Fig. 29 represents a round piece of glass, to which is fitted a concave glass, for the purpose of confining animalcules, and other small living creatures, for minute inspection.

Compound microscopes have been much improved of late, by using small achromatic lenses for the object glasses. They have been made as small as 1/5, 1/4, 1/2, and 1 inch focal distance, and sometimes two or three of them are occasionally combined together, which produces a very high magnifying power, with great distinctness. But such lenses add considerably to the expense. About ten years ago £1. 3 s. was


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the cost of one of these lenses 1 1/4 inch focal distance, and those which were of a shorter focal distance were charged at two guineas and upwards. A person who feels himself unable to purchase an expensive compound microscope, may construct a pretty powerful one for a few shillings, by attending to the following directions:--Procure for the object glass a lens about 1/2 an inch focal distance, another for the amplifying lens 2 1/2 inches focal length and 1 1/2 inch diameter, and a third glass 1 inch focal distance, to be placed next the eye. The distances at which these glasses should be placed from each other are as follows:--The object glass, 1/2 inch focal distance, should be placed at the end of a small tube next the object, and the aperture or hole that lets in the light should not exceed one-tenth of an inch in diameter. At the distance of about 7 inches from this glass the amplifying lens should be placed, and the glass next the eye, 1 inch focal distance, should be placed about 1 3/4 inch from the amplifying lens. Such a microscope, reckoning the combined eye glasses to magnify the image 6 times, and the object glass to magnify the object 14 times, will produce a magnifying power of 84 times in lineal dimensions, and in


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surface 7,056 times--a power which will show a small creature, such as a flea, as if it were 8 1/2 inches long and of a corresponding breadth, and will bring to view all the larger species of animalcules. The stage and its supports may be made of wood, and the tubes of paper or very thin pasteboard. The tube 5, in which the eye glasses are placed, should be made so as to pull out occasionally, to increase the distance between the eye glasses and the object glass, and consequently the magnifying power. Any person with mechanical talent can easily make such an instrument at a trifling expense. The compound microscope is more pleasing in its use than the single microscope; it has a larger field of view, and the eye is not so much strained as in looking through very small lenses.

3. The solar microscope. --This microscope is constructed in the following manner. In a closed window shutter, or in a board fitted into the window, make a hole about 3 inches in diameter, through which the sun may cast a cylinder of rays, A A, into the darkened room, fig. 30. Into this hole place the end of a tube, containing two convex glasses and an object, namely, a convex glass, a a, of about 2 inches


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diameter and 3 inches focal distance, is to be placed in the end of the tube, which is put into the hole. The object, b b, is placed about 2 1/2 inches from the glass a a. If the object be a living animal it must be put between two concave glasses. A little more than a quarter of an inch from the object is placed the small convex lens c c, whose focal distance may be about a quarter of an inch. The tube may be
Fig. 30.

[Illustration : A diagram showing the use of a mirror and two double convex lenses to create a large projected image of a small object. The parts of the diagram are labelled with letters.]
so placed, when the sun is low, as that his rays, A A, may enter directly into it; but when he is high, his rays, B B, must be reflected into the tube by the plain mirror C C. Things being in this state, the rays that enter the tube will be coveyed by the lens a a towards the object b b, by which means it will be strongly illuminated, and the rays d, which flow from it through the lens c c, will form a large inverted picture of the object at D D, which being received


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on a white screen, will represent the object magnified in length, in proportion of the distance of the picture from the glass c c to the distance of the object from the same lens. Thus, suppose the distance of the object from the lens to be half an inch, and the distance of the image 14 feet, or 168 inches, the object will be magnified in length and breadth 336 times, and in surface 112,896 times.

Fig. 31.

[Illustration : An illustration of a solar microscope, fully assembled. The parts of the microscope are labelled with letters.]

In fig. 31, some of the parts of this instrument


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are more particularly represented. The square plate, b c d, is attached to the window shutter by the screws e f. The mirror g is mounted on a wooden frame, and may be elevated or depressed by a screw at d. A rotary motion is communicated by a pinion and handle at c. The first lens is placed in the tube a, immediately adjoining the mirror. Another tube, m, is attached by a screw at n, and contains the small lenses for magnifying the object, and the rack-work k l for adjusting the focus of the instrument. The objects are introduced at i. When lenses of high power are employed at h, they are now constructed on the achromatic principle.

This instrument is not so much used as formerly, in consequence of the invention of the oxyhydrogen microscope, which is not dependent on the sun, but may be used either by day or by night, provided the room be darkened, the oxyhydrogen light being substituted for that of the sun. This is the microscope which is now exhibited in lecture-rooms, and in our Polytechnic Institutions. It may not be improper here, to give a hint to some persons not much acquainted with such exhibitions. One of the objects shown by this microscope (the


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oxyhydrogen) is the appearance of a heterogeneous mass of animals, which appear to be fighting with each other on a very large screen, intermixed with vegetable fibres. It has been taken for granted by some spectators, that these animals were all contained in a drop or two of water, and that they consorted together in the manner represented. This is by no means the case; no such associations are to be found in the animalcular world. A number of small animals of different kinds, most of them visible to the naked eye, are collected by the exhibitors, and put into a small glass vessel, perhaps an inch or two in diameter, along with water and a few vegetable fibres. It forms a striking exhibition, but persons should beware of deducing from it erroneous conclusions.

4. The lucernal microscope. --This instrument was invented by Mr. George Adams, an optician in London. It consists of a hollow pyramidal box of mahogany, which forms the body of the microscope. Fig. 32 exhibits a view of this instrument, mounted to examine opaque objects. b is the large pyramidal box, supported firmly on the brass pillar n, by means of the socket m and the curved piece e. a is a


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guide for the eye, to direct it in the axis of the lenses; it consists of two brass tubes l, one sliding within the other, and a vertical flat piece, at the top of which is the hole for the eye. The inner tube may be pulled out or pushed in, to adjust it to the focus of the glasses. The
Fig. 32.

[Illustration : An illustration of a lucernal microscope mounted on a stand. The parts of the microscope are labelled with letters.]
vertical piece may be raised or depressed, that the hole through which the object is to be viewed may coincide with the centre of the field of view. At the small end of the cone is placed a tube which carries the magnifiers, one of which is represented at c ; the tube may be unscrewed


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occasionally from the wooden body. Beneath the cone is placed a long square bar, which passes through, and carries the stage or frame that holds the objects; this bar may be moved backwards or forwards, to adjust it to the focus, by means of the pinion k. A handle, with an universal joint for turning the pinion, is shown at o. The stage h, for opaque objects, fits upon the bar by means of a socket, and is brought nearer to or further from the magnifying lens by turning the pinion k. At the lower part of the stage there is a semicircular lump of glass, g, which is designed to receive the light from the lamp, and to throw it on the concave mirror f, whence it is reflected on the object. The upper part of the opaque stage takes out, that the stage for transparent objects may be inserted in its place. Between the exterior of the two lenses at the larger end and the eye of the observer, there is placed a plate of glass, rough ground on one side, which serves as a screen to receive the rays of light proceeding from the object whose representation is to be viewed. An Argand lamp, or the oxyhydrogen light, is placed beyond the object, before the glass lump g.

By this instrument, opaque objects may be


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seen with ease and distinctness. The beautiful colours with which most of them are adorned are rendered more brilliant, without changing in the least the real tint; and the concave and convex parts retain also their proper form. The facility with which all opaque objects are applied to this instrument is another considerable advantage, and one almost peculiar to it. The lucernal microscope does not in the least fatigue the eye; the object appears like nature itself, giving ease to the sight and pleasure to the mind; and there is no occasion to shut the eye that is not directed to the object. The outlines of every object may be taken even by those who are not accustomed to draw, while those who can draw well will receive great assistance. Transparent objects as well as opaque may be copied in the same manner. This instrument may be used at any time of the day, but the best effect is by night, in which respect it has a superiority over the solar microscope, which can only be used when the sun shines. Such are some of the properties of the lucernal microscope, as stated by the inventor.

Besides the microscopes we have already


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described, different forms of this instrument have been constructed on the principle of reflection, by a combination of speculums both convex and concave. These microscopes were constructed as early as the year 1738, by Dr. Smith, Mr. Baker, and others, but they had been abandoned for many years, till, in the year 1815, Amici, a Frenchman, directed his attention to their construction, and greatly improved them; and they were still further improved in England by Dr. Goring and Mr. Cuthbert; but owing to the difficulty in constructing the reflectors, and the great trouble in managing them, they again fell into disuse, and even Amici himself returned to his former experiments with achromatic object glasses.

If it be asked which of the microscopes now described we would recommend for making researches into the minute parts of nature, we answer, without hesitation, the compound microscope furnished with achromatic object glasses. The compound microscope as now improved, to use the words of an eminent optician, "has, within the last sixteen years, been elevated from the condition in which it was previously found, to that of being the most important


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instrument ever yet bestowed by art upon the investigator of nature." The application of achromatic object glasses to compound microscopes has only been attempted within the last twenty-six years. In 1824, the late Mr. Tulley, of London, succeeded in making the first English achromatic object glass for a compound microscope. It was composed of three lenses, and was capable of transmitting a pencil of rays of 18°. He soon after constructed another combination, to be placed in the front of the first mentioned, which increased the angle of the pencil to 38°. Mr. Tulley's object glass exhibited a flat field, and was perfectly corrected; to it was applied an eye-piece, by which the magnifying power produced was 120 diameters, but when the second combination was added, the power was increased to 300. These object glasses have since been improved by Messrs. Lister, Powell, Ross, and Smith. Their focal distances vary from 1 1/2 inch to 1/8th of an inch and they may be used either separately or in combination. Sometimes three sets of them are combined together, which produces a very powerful effect. Magnifying powers equal to 1,200 diameters have thus been obtained with great distinctness, that is, the surfaces of object


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have been magnified one million four hundred and forty thousand times. Some of the powers thus obtained have been equal to even 2,000 diameters, and consequently the surface magnified four million times. But such instruments, as formerly stated, are expensive.

A NIMALCULES. --This term is now generally used to distinguish animals of a size so diminutive that their true figure cannot be discerned without the assistance of glasses; and more especially, it is applied to such as are altogether invisible to the naked eye. By the microscope we are brought into acquaintance with new tribes of the living world, and innumerable animated beings, which, from their minuteness, would without it have escaped our observation. How many of these invisible tribes there may be throughout the air, the waters, and the earth, is still unknown, but they doubtless far exceed the number of all other classes of living creatures combined. To know that there are myriads of atoms, endued with vitality, existing in a single drop of water, executing all their


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various functions and evolutions with as much rapidity and ease as if the range afforded them were boundless as the ocean, must powerfully interest every mind which takes pleasure in the works of God.

It is almost impossible to convey a correct idea of the various shapes of these singular forms of life to those who have not actually beheld them. They appear to have little or no similarity to the other diversified orders of animal existence. Some of the smallest appear merely like moving points or atoms; the large ones exhibit an astonishing variety; some are like spheres, others are egg-shaped, some are like hand-bells, others are like wheels turning on an axis; some represent fruits and vegetables of various kinds, others resemble eels, serpents, and snakes; some are like double-headed monsters, and others like cylinders; some have the appearance of funnels, tops, pitchers, and flasks; others are worm-like; some have horns, fins, and feet; others resemble small fishes, playing in the rivers or the sea; some are like long hairs, a hundred times longer than they are broad; others are like spires and cupolas; some of them are almost


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visible to the naked eye; others so small that a human hair would cover more than a hundred of them; while millions of millions of them might be contained within the compass of a square inch. They, however, possess peculiar habits, adapted to their respective forms. While some move through the water with the greatest rapidity, darting, leaping, or swimming, others creep or glide along, and many are so passive, that it requires patient observation to discover any of their movements. We may now give a brief description of some individual species of animalcules.

The phenomena of blood.

The blood of man and of land animals is found by the microscope to consist of round red globules, floating in a transparent water or serum. Each red globule is made up of six smaller and more transparent ones, and each of these again, according to Leuwenhoek, is composed of six globules still more minute and colourless, so that every common red globule is compounded of at least thirty-six smaller


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ones. The diameter of a common round globule of human blood has been found to be equal to the 1/1940th part of an inch. In order to view the blood with a microscope, take a small drop of warm blood as it comes from the vein, and, with the tip of a hair pencil, spread it as thinly as possibly on a slip of glass. It may also be diluted with a little warm water.

Description of the parts of some small animals, as seen in the microscope.

Vegetable substance.

Vegetable nature presents to us an immense field for the employment of the microscope, and innumerable beauties and wonders which, without this instrument, would never have been discovered or imagined. Our limits prevent us from doing more than briefly noticing a few examples. We may notice--

A great number of other microscopic objects might have been described, had our limited space permitted, but we shall briefly mention only the following. By the microscope it has been discovered


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that there are scales on the human skin, so small that, according to the calculation of Leuwenhoek, 200 of them may be covered by a grain of sand. The perspirable matter is supposed to issuebetween these scales, which lie over the pores through which the watery humours exude. The pores of the skin are exceedingly numerous. If a slice of the upper skin be cut off with a sharp razor, as thin as possible, and a second slice from the same place be immediately applied to the microscope, innumerable pores will be perceived. M. Leuwenhoek supposes that there are 100 pores in a line one-tenth of an inch long. An inch will, therefore, contain 1,000 in a row, and a foot 12,000. According to this computation, a foot square must have in it 144 millions of pores; and if the superficies of a middle-sized man be 14 square feet, there will be in his skin 2,016 millions of pores. If the hand be well washed, and examined with a magnifying lens in the palm, or upon the ends or the first joints of the thumb and fingers, innumerable ridges, parallel to each other, will be found. On these ridges, pores may be perceived, lying in rows; and when viewed through a good glass, every pore seems a little fountain, with the perspiration


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standing in it as clear as water; if wiped away, the perspiration will be found immediately to spring up again.

The threads with which a spider weaves its web are worthy of our attentive consideration, on account of their extreme fineness. M. Leuwenhoek, having dissected the body of the longest spider he could find, and nicely examined each part, at last discovered a vast number of instruments, as they may be termed, from which the spider draws threads of various fineness; these thread vessels he judged to be at least 400, lying, not close together, but in several distinct clusters. Having laid a spider on its back, so that it could not stir, he pulled out, with a very fine pair of pincers, a thread that he perceived sticking out of one of the working instruments. At the same time he saw a great many other very fine threads issuing from the insect's trunk, which, at the distance of a hair's breadth or two from the body, were joined together, and made thick threads. Some of these filaments were so extremely fine as almost to elude his sight, though he made use of his most powerful glasses. Hence it appears that the threads of a spider's web, which to the naked eye seem to be single, do really consist of several plies, some of which


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are so exceedingly fine, that Leuwenhoek thought that 100 of them put together would not make one-hundredth part of the thickness of a hair of his head. Ten thousand, therefore, of the fine threads of a full-grown spider are not so thick as a human hair; and if we add to this, that 400 young spiders, when they first begin to spin, are not, one with another, bigger than a full-grown one, and yet that each of these spiders is provided with all the instruments of the old one, it will follow that the smallest thread of a young spider is 400 times smaller than that of a great one; and if so, then 4,000,000 of such threads are not so thick as the hair of a man's head, a fineness quite astonishing, and beyond our conception.

Test objects. --About the year 1826, Dr. Goring discovered that the structure of certain bodies could be readily seen by some microscopes, but not by others. These bodies he called test objects. In the course of his experiments, he was led to the conclusion that there were two distinct powers in a microscope, namely, defining and penetrating, and that an object glass might possess the one almost to perfection, and yet be totally devoid of the


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other, or might be perfect in both. At present, however, it is the opinion of the most celebrated opticians that the terms definition and defining power are the only expressions requisite to be employed to denote the good or bad qualities of any microscope. The test objects now generally employed for ascertaining the merits of any achromatic combination may be divided into three kinds, namely, hairs of animals, scales from the wings and bodies of insects, and the siliceous coatings of recent and fossil infusoria, those of the latter kind being the most difficult of all to define. The following list contains a few of the test objects to which allusion has been made:--

Comparison of the works of nature and art. --There is nothing that more conspicuously displays the perfection of the works of God than a comparison of them with the finest works of art. The contrast between the one and the other is exceedingly striking, and humbling to the pride of man. His best performances, when examined by the microscope, appear coarse and shapeless; but the more closely and clearly we are enabled to inspect the works of God, the more apparent is their supreme excellence. This may be exemplified by a few illustrations. The point of an exceedingly


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small needle, highly polished, when viewed with a high magnifying power, appears neither round nor flat, but full of holes and scratches, and as broad and blunt as the end of a poker, and looks as if it had been hammered on the anvil. On the other hand, the sting of a bee or of a gnat, the proboscis of a butterfly or flea, appear, when examined by the microscope, to be formed with the most surprising beauty, and with exact regularity. The sting of a bee shows a polish without the least flaw, blemish, or inequality, and ends in a point too fine to be discerned; yet this is only the sheath of instruments still more refined. The adjoining figure represents a piece of exceedingly fine lawn, as it appeared through the microscope. From the great distances between its threads it looks like a lattice, and the threads themselves appear coarser than ropes. Compare this with any leaf of the forest, or grass of the field, under the same magnifying power, and a most striking contrast will be perceived.

The small dot or point, which is generally the mark of a full stop or period, when greatly magnified appears to be rough, jagged, and uneven all round its edges, and very far from being truly round. The smoothest and most


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exquisitely engraved lines and points, when examined by the microscope, seem like so many furrows and holes, or like daubings on a mat made with a blunt extinguished brand. On submitting to the microscope the edge of a very keen razor, it appears as broad as the back of a thick knife, rough, full of notches, and furrows, sharper in some places than others. Compare these works of human art with the operations of a puny insect, directed by instinct,


[Illustration : An illustration of fine, interwoven threads.]
or the wisdom and intelligence of its Creator, and the contrast appears most striking and admirable. A silkworm's web, examined by the microscope, appears perfectly smooth and shining, everywhere equal, and as much finer than any thread which the most dexterous spinner in the world can make, as the thinnest twine


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is smaller than the thickest cable. The web woven by a spider is still more delicate. Every thread of it is many thousands of times smaller than a human hair. In short, every minute hair and fibre on a flea, a gnat, a fly, and other insects, is not only smooth and beautifully polished, but, when magnified thousands of times, appears as sharp at the point as a needle.

Thus sink the works of man when placed in comparison with the works of God. Wherever the microscope is applied, there beauty, order, and perfection are displayed. In all the innumerable varieties of insects which fly through the air, swim in the waters, or crawl along the earth, symmetry, proportion, and uniformity are perceptible. Their bodies, heads, wings, feet, and other members, are embellished as with rubies, diamonds, gold, silver, and pearls; they are coloured with azure, green, yellow, red, and vermilion hues; yet in all there is nothing gaudy or incongruous. How unutterable the Perfection of which these are the creations!

Method of using microscopes. --1. In applying the microscope to use, examine in the first place whether the glasses be clean; if not, they


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must be carefully wiped with very soft leather, taking care not to soil them with the fingers, nor to place them in an oblique situation. 2. The object should be brought as near the centre of the field of view as possible, for there only it will be exhibited in perfection. In a compound microscope, the eye should be moved up and down till the situation is found where the largest field and the most distinct view of the object are to be had. 3. A small magnifying power should always be first used, by which means an observer will best obtain an exact idea of the connexion and situation of the whole. In general, it may be remarked, that there is no advantage in examining any object with a higher power than what shows it distinctly. A moderate power affords a greater light, and shows objects more clearly, than the highest powers, although sometimes these are required. 4. The eye should be protected from all extraneous light, and should not received any but what is transmitted through or reflected from the object. 5. According to sir D. Brewster, the best position for microscopical observations is when the observer is lying horizontally on his back. This, he says, arises from the perfect stability of his head, and from


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the equality of the lubricating film of fluid which covers the cornea. The worst of all positions is that in which we look downwards, vertically. If we stand up and look horizontally, parallel markings or lines will be seen most perfectly when their direction is vertical, being the course in which the lubricating fluid descends over the cornea. 6. Every part of the view should be excluded except that which is under immediate observation. 7. In every case of microscopical observations, homogeneous yellow light, procured from a monochromatic lamp, should be employed. 8. In viewing opaque objects, an opportunity should be taken when the sun shines of making his rays strike upon the surface of the object. In this way we may view to advantage the body and the eye of a fly, the fibres of a peacock's feather, and similar objects. Or, a broad convex lens, about three inches focal distance, may be so placed as to throw the light of a lamp upon the object, the surface of which would then be distinctly visible. The best mode, however, of viewing opaque objects is by means of a contrivance called the Lieberkuhn, from the name of its inventor. This consists of a concave silver speculum, highly polished, in the centre of


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which is placed a magnifying lens. This speculum has a small hole in its centre, and may be applied immediately below the object glass of a compound microscope. For this purpose the speculum must be placed at the end of a small tube, to slip over the tube which holds the object glass. The light proceeding from the mirror falls upon the speculum, and is reflected perpendicularly on the upper surface of the object.