With respect to the accessory parts, they consist chiefly of the eyelids, which serve to wipe the face of the eye, and protect it from accidents and dust; the lachrymal apparatus, which serves to wash it with tears, so as to keep it continually brilliant; and the muscles, requisite to direct it upon any point.

Of the nervous part of the eye, so far as its functions are concerned, but little is known. The retina receives the impressions of the light, which are conveyed along the optic nerve to the brain.

Now, as respects the optical action of the eye, it is obviously nothing more than that of a convex lens, with which, indeed, its structure actually corresponds; and as in the focus of such a convex lens objects form images, so by the conjoint action of the cornea and crystalline, the images of the things to which the eye is directed form at the proper focal distance behind-that is, upon the retina. Distinct vision only takes place when the cornea and the lens have such convexities as to bring the images exactly upon the retina.

In early life it sometimes happens that the curvature of these bodies is too great, and the rays converging too rapidly, form their images before they have reached the posterior part of the eye, giving rise to the defect known as short-sightedness-a defect which may be remedied by putting in front of the cornea a concave glass lens of such concavity as just to compensate for the excess of the convexity of the eye.

In old age, on the contrary, the cornea and the lens become somewhat flattened, and they cannot converge the rays soon enough to form images at the proper distance behind. This long-sightedness may be remedied by putting in front of the cornea a convex lens, so as to help it in its action.

Concave or convex lenses, thus used in front of the eyes, constitute spectacles. It is believed that this application was first made by Roger Bacon, and it unquestionably constitutes one of the most noble contributions which science has ever made to man. It has given sight to millions who would otherwise have been blind.*

The image which is formed by a convex lens being inverted as respects its object, so must the images which form at the bottom of the eye. It has, therefore, been a question among optical writers, why we see objects in their natural position, and also why we do not see double, inasmuch as we have two eyes. Various explanations of these facts have been offered, chiefly founded upon optical principles. None, however, appear to have given general satisfaction, and in reality, the true explanation, I believe, will be found not in the optical, but in the nervous part of the visual organs. It is no more remarkable that we see single, having two eyes, than that we hear single, having two ears. It is the simultaneous arrival in the brain that gives rise out of two impressions to one perception, and accordingly, when we disturb the action of one of the eyes by pressing on it, we at once see double. Among the peculiarities of vision it may be mentioned, that for an object

* A defect, and not an uncommon one, with some persons, consists in the eyes refracting the rays of light with different powers in different places. The defect may be detected by making a small pin-hole in a card, which is to be moved from close to the eye to an arm's length, while the gaze is directed toward the sky, or some bright object. If the sight be perfect the hole does not alter its circular form at all; but if the peculiar defect exists, the hole becomes elongated, and ultimately merges into a straight line. M. Airey considers that a spherical cylindrical lens will correct the defect, as it succeeded in his own case.-ED.

to be seen it must be of certain magnitude, and remain on the retina a sufficient length of time; and, for distinct vision, must not be nearer than a certain distance, as eight or ten inches. This distance of distinct vision varies somewhat with different persons. The eye, too, cannot bear too brilliant a light, nor can it distinguish when the rays are too feeble; though it is wonderful to what an extent, in this respect, its powers range. We can read a book by the light of the sun or the moon; yet the one is a quarter of a million times more brilliant than the other. Luminous impressions made on the retina last for a certain space of time, varying from one-third to one-sixth of a second. For this reason, when a stick, with a spark of fire at the end, is turned rapidly round, it gives rise to an apparent circle of light.

By accidental or physiological colours we mean such as are observed for a short time depicted on surfaces, and then vanishing away. Thus, if a person looks steadfastly at a sheet of paper strongly illuminated by the sun, and then closes his eyes, he will see a black surface corresponding to the paper. So if a red wafer be put on a sheet of paper in the sun, and the eye suddenly turned on a white wall, a green image of the wafer will be seen. Spectral illusions in the same way often arise-thus, when we awake in the morning, if our eyes are turned at once to a window brightly illuminated, on shutting them again we shall see a visionary picture of every portion of the window, which after a time fades away.

CHAPTER XLVI.

OF OPTICAL INSTRUMENTS.

The Common Camera Obscura-The Portable Camera-The Single Microscope-The Compound Microscope-Chromatic and Spherical AberrationThe Magic Lantern-The Solar Microscope-The Oxihydrogen Micro

scope.

In this and the next chapter I shall describe the more important optical instruments. These, in their external appearance, and also in their principles, differ very much according to the taste or ideas of the artist. The descriptions here given will be limited to such as are of a simple kind.

THE CAMERA OBSCURA, or dark chamber, originally consisted of nothing more than a double convex lens of a foot or two in focus, fixed in the shutter of a dark room. Opposite the lens, and at its focal distance, a white sheet received the images. These repre

sent whatever is in front of the lens, giving a beautiful picture of the stationary and movable objects in their proper relation of light and shadow, and also in their proper colours.

In point of fact, a lens is not required for if, into a dark chamber, C D, Fig. 280, rays are admitted through a small aperture,

Fig. 280.

M

chamber, of whatever objects are in front. Thus the object, A B, gives the inverted image, b a. These images are, however, dim, owing to the small amount of light which can be admitted through the hole. The use of a double, convex lens permits us to have a much larger aperture, and the images are correspondingly brighter.

b

The portable Camera Obscura consists of an achromatic double convex lens, a a', set in a brass mounting in the front of a box consisting of two parts, of which c c' slides in the wider one, b b'. The total length of the box is adjusted to suit the focal distance of the lens. In the back of the part, c c', there is a square piece of ground glass, d, which receives the images of the objects to which the lens is directed, and by sliding the movable part in or out, the ground glass can be brought to the precise focus. The interior of the box and brass piece, a a', is blackened all over to extinguish any stray light.

Fig. 281.

The images of the camera are, of course, inverted; but they can be seen in their proper position by receiving them on a looking-glass, placed so as to reflect them upward to the eye. Objects that are near, compared with objects that are distant, require the back of the box to be drawn out, because the foci are further off. Moreover, those that are near the edges are indistinct, while the central ones are sharp and perfect. This arises from the circumstance that the edges of the ground glass are further from the lens than the central portion, and therefore out of focus.

OF MICROSCOPES.

The Single Microscope.-When a convex lens is placed between the eye and an object situated a little nearer than its focal distance, a magnified and erect image will be seen.

The single microscope consists of such a lens, m, Fig. 282, the object, bc, being on one side, and the eye, a, at the other, a magnified and erect image, B C, is seen.

The

Fig. 282.

linear magnifying power of such a lens is found by dividing the distance of distinct vision by its focal length.

The Compound Microscope commonly consists of three lenses, A B, E F,

Fig. 283.

course, seen in an inverted position. cept the extreme pencils of light, nm,

C D, Fig. 283, A B being the object-glass, E F the field-glass, and CD the eye-glass. Beyond the object-glass is placed the object, at a distance somewhat greater than the focal length; a magnified image is, therefore, produced, and this being viewed by the eye-glass is still further magnified, and, of The use of the field-glass is to intercoming from the object-glass, which

would otherwise not have fallen on the eye-lens. It therefore increases the field of view, and hence its name.

In this instrument the object-glass has a very short focus; the eye-glass, one that is much larger; and the field-glass and the eye-glass can be so arranged as to neutralize chromatic aberration.

To determine directly the magnifying power of this instrument, an object, the length of which is known, is placed before it. Then one eye being applied to the instrument, with the other we look at a pair of compasses, the points of which are to be opened, until they subtend a space equal to that under which the object appears. This space, being divided by the known length of the object, gives the magnifying power.

B

In Fig. 284, we have a representation of the compound microscope, as commonly made. A B is a sliding brass tube, which bears the eye-glass; mn is the object-glass; I K the field-glass; ST a stage for carrying the objects. It can be moved to the proper focal distance by means of a pinion. At V there is a mirror which reflects the light of a lamp or the sky upward, to illuminate the object. The body of the microcope is supported on the pillar M, and it can be turned into the horizontal or any oblique position to suit the observer, by a joint, N. To the better kind of instruments micrometers are attached, for the purpose of determining the dimension of objects. These are sometimes nothing more than a piece of glass, on which fine lines have been drawn with a diamond, forming divisions the value of which is known. Such a plate may be placed either immediately beneath the object or at the diaphragm, which is between the two lenses.

In microscopes the defective action of lenses, known as chromatic aberration, and described in Chapter XL., interferes, and, by imparting prismatic colours to the edges of objects, tends to make them indistinct. To overcome this difficulty, achromatic object-glasses are used in the finer kinds of instruments.

Fig. 284.

Besides chromatic aberration, there is another defect to which lenses are subject. It arises from their spherical figure, and hence is designated spherical aberration. Let P P, Fig. 285, be a convex lens, on which rays, Ê N, EN, EM, EM, E A, from any object, E e, are incident, it is obvious that the

M
A

M

principal ray, EA, will pass on through B to F without undergoing refraction. Now, rays which are near to this, as E M, E M, converge by the action of the lens to a focus at F: but those which are more distant, and fall near the edges of the lens, as EN, EN, converge more rapidly, and come to a focus at G. Thus images, Ff, G g, are formed by the extreme rays, and an interme.. diate series of them by the intermediate rays, the whole arising from the peculiarity of figure of the lens. It is, indeed, the same defect as that to which spherical mirrors are liable, as explained in Chapter XXXVI.; and

Fig. 285.

hence, to obtain perfect action with a spherical lens, as with a spherical mirror, its aperture must be limited.

OF OPTICAL INSTRUMENTS.

THE MAGIC LANTERN consists of a metallic lantern, A A' Fig. 286, in front of which two lenses are placed. One of these, m, is the illuminating lens, the other, n, the magnifier. A powerful Argand lamp is placed at L, and behind it a concave mirror, p q.

In the space between the two lenses the tube is widened, cd, or such an arrangement made that slips of glass, on which various figures are painted, can be introduced. The action of the instrument is very simple. The mirror and the lens m illuminate the drawing as highly as possible; for the lamp being placed in their foci, they throw a brilliant light

Fig. 286,

upon it, and the magnifying lens, n, which can slide in its tube a little backward and forward, is placed in such a position as to throw a highly magnified image of the drawing upon a screen, several feet off, the precise focal distance being adjusted by sliding the lens. As it is an inverted image which forms, it is, of course, necessary to put the drawing in the slide, c d, upside down, so as to have their images in the natural position. Various amusing slides are prepared by the instrument-makers, some representing bodies or parts in motion. The figures require to be painted in colours that are quite transparent.

[By employing two distinct magic lanterns, or two lanterns inclosed in the same case, the dissolving views are exhibited.]

THE SOLAR MICROSCOPE.-This instrument, like the magic lantern, consists of two parts-one for illuminating the object highly, and the other for

W

Fig. 287.

magnifying it. It consists of a brass plate, which can be fastened to an aperture in the shutter of a dark room, into which a beam of the sun may be directed by means of a plane mirror. In Fig. 287, M is the mirror, to which movement in any direction may be given by the two buttons, X and Y, that rays from the sun may be reflected horizontally into the room. They pass through a large convex lens, R, and are converged by it; they again impinge on a second lens, US, which concentrates them to a focus, the precise point of which may be adjusted by sliding the lens to the proper position by the button B. PP' is an apparatus consisting of two fixed plates, with a movable one, Q, between them, Q being pressed against P' by means of spiral springs. This apparatus is for the purpose of supporting the various objects which are held by the pressure of Q against P. Immediately beyond this, at L, is the magnifying lens, or object-glass, which can be brought to the proper position from the