CHAPTER II.

THE EYE AS AN OPTICAL INSTRUMENT.

SECTION I.-THE NORMAL EYE.

THE further explanation of the wonderful mechanism of the eye is best brought out by a comparison with some optical instrument. We select for this purpose the photographic camera. The eye and the camera: the one a masterpiece of Nature's, the other of man's work.

We pass over, with bare mention, some obvious resemblances, in which, however, the superiority of the eye is evident: such, e. g., as the admirable arrangement of the lids for wiping and keeping bright while using, and for covering when not in use; also, the admirable arrangement of muscles, by which the eye is turned with the greatest rapidity and precision on the object to be imaged, so superior to the cumbrous movement of the camera for the same purpose. We pass over these and many other minor points to come at once to the main points of comparison.

Take, then, the eye out of the socket-the dead eyeand the camera without its sensitive plate-with only the insensitive ground-glass receiving plate. They are both now pure optical instruments, and nothing more. They are both contrived for the same purpose, viz., the formation of a perfect image on a screen properly placed.

Look into the camera from behind, and we see the inverted image on the ground-glass plate; look into the eye from behind, and we see also an inverted image on the retina. The end, therefore, is the same in the two cases. We now proceed to show that the means by which the end is attained are also similar.

1. The camera is a small, dark chamber, open to light only in front, to admit the light from the object to be imaged. It is coated inside with lampblack, so that any light from the object to be imaged or from other objects which may fall on the sides will be quenched, and not allowed to rebound by reflection, and thus fall on the image and spoil it. No light must fall on the image except that which comes directly from the object. So the eye also is a very small, dark chamber, open to light only in front, where the light must enter from the object to be imaged, and lined with dark pigment, to quench the light as soon as it has done its work of impressing its own point of the retina, and thus prevent reflection and striking some other part, and thus spoiling the image.

2. Both camera and eye form their images by means of a lens or a system of lenses. The manner in which these act in forming an image has already been explained (page 21). It is precisely the same in both cases. But lenses which form a perfect image are very difficult of construction. There are, especially, two main imperfections which must be corrected, viz., chromatism and aberration.

3. Correction of Chromatism.-In the image formed by a simple, ordinary lens, all the outlines of figures are found to be slightly edged with rainbow hues. If we look through such a lens at an object, the outlines of the object will be similarly edged with colors, especially

if the object lie near the margin of the field of the lens. This is explained as follows:

Ordinary sunlight, as every one knows, consists of many colors mixed together, the mixture producing the impression of white. If a beam of sunlight be made to pass through a glass prism, the beam is bent: but more, the different colors are unequally bent, so that they are separated and spread out over a considerable space. This colored space is called the spectrum. In Fig. 9 the

r-v, spectrum: r, red; o, orange; y, yellow; g, green; b, blue; i, indigo; v, violet.

straight beam, a b, is bent by the prism so as to become a cd; this is called refraction. But also the different colors are unequally bent; red is bent least and violet most, the other colors lying between these extremes; thus they are spread out over a considerable colored space. This unequal refraction is called dispersion. If we look through a prism at objects, we will find that the outlines of the objects will be edged with exactly similar colors. Now all refraction is accompanied by dispersion; therefore a simple, uncorrected lens always disperses, especially on the edges where the refraction is greatest; and, therefore, also, the images made by such a lens will be edged with color. Thus the light from the radiant a (Fig. 10), being white light, is dispersed; the violet rays, being more bent, reach a focus at a',

but the red only at a", the other colors at intermediate points. There is, therefore, no place where all the rays from the radiant come to a focus-there is no commcn focal point for the radiant a. The best place

FIG. 10.

for the receiving screen would be SS, but even here there is no perfect focus. Evidently, therefore, the conditions of a perfect image are not fulfilled. This defect must be corrected. It is corrected in every good lens.

FIG 11.

In order to understand how this is done, it must be remembered, first, that concave and convex lenses antagonize, and, if of equal refractive power, neutralize each other. Therefore, a combination of a double convex and a double concave lens, if of same material and of equal curvature, like Fig. 11, A, will produce no refraction, because the refraction produced in one direction by the convex lens is completely destroyed by refraction in the opposite direction by the concave lens. Such a combination will therefore make no image. In order that such a combination should make an image at all, it is necessary that the convexity should predominate over the concavity, as in Fig. 11, B. Again, it must be remembered that dispersion is not always in proportion to refraction. Some substances

B

have a higher refractive power and a comparatively low dispersive power, and vice versa. This is the case with different kinds of glass.

Now, suppose we select a glass with excess of refractive over dispersive power for our convex lens, and one with excess of dispersive over refractive power for our plano-concave lens (Fig. 11, B), and cement these together as a compound lens: it is evident that these may be so related that the plano-concave lens shall entirely correct the dispersion of the convex lens without neutralizing its refraction, and therefore the combination will be a refractive, but not a dispersive, lens, and therefore will make an image without colored edges. Such a compound lens is called achromatic.

1

This is the way in which art makes achromatic lenses, and all good optical instruments have lenses thus corrected. Now, the lenses of the eye are apparently corrected in a similar manner. The eye consists of three lenses the aqueous, the crystalline, and the vitreous. These have curvatures of different kinds and degrees: the aqueous lens is convex in front and concave behind; the crystalline is bi-convex; the vitreous is concave in front. As its convex outer surface can not be regarded as a refracting surface, since this is in direct contact with the screen to be impressed, it may be considered as a plano-concave lens. The refractive powers of the material of these are also different: that of the crystalline being greatest, and the aqueous least. The dispersive powers of these have not been determined, but they probably differ in this respect also. Thus, then, we have here also a combination of different lenses, of different curvatures, and different refractive, and probably dispersive, power, and for the same purpose, viz., correction of chromatism. It is an interest