from points A and B of the object, which enter the pupil, are refracted by the lenses of the eye, and brought to a focus on the retinal screen at a' b'. Now, since the rays from every intermediate point of the object will be similarly focused, we will have a perfect image of the object painted on the retina.

This fundamental fact may be proved in many ways by observations on the dead eye: 1. If the eye of an ox be taken from the socket, and the sclerotic carefully removed, so that the back parts of the eye are somewhat transparent, a miniature image of the landscape may be seen there; or, 2. If we remove the eyeball of a white rabbit, we will find that, on account of the absence of black pigment in the choroid of these albinos, the transparency of the coats of the eye enables us to see the image, even through the sclerotic, or much more distinctly if the sclerotic be removed; or, 3. We may remove all the coats of the dead eye and replace them by a film of mica-the image will be very distinct; or, 4. The image may be seen in the living eye by means of the ophthalmoscope.

By reference to the diagram, Fig. 8, it is seen that the central rays from all radiants cross each other in the lens. This point of ray-crossing is called the nodal point. It is a little behind the center of the lens.

CHAPTER II.

THE EYE AS AN OPTICAL INSTRUMENT.

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 human

art.

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 28). 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

с

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

rv, 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 focus-there is no The best place

common focal point for the radiant a.

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