tion as we get brightness do we lose distinctness. We can not get both at the same time.

Experiment.-Let a room with solid shutters be darkened; let one shutter have a hole of a few inches in diameter; cover the hole with an opaque plate of sheet iron, in which there is a very small hole, one tenth to one twentieth of an inch in diameter. If, now, a sheet of white paper be held a little way from the small hole, an inverted image of the external landscape will be seen on the sheet. If we increase the size of the hole, the image will be brighter, but also more blurred.

Illustrations. Many simple experiments may be made illustrating this principle. A pinhole in a card will make an inverted image of a candle flame. When the sun is in eclipse, it may be examined without smoked glass, by simply allowing it to shine through a pinhole in a card upon a suitable screen. In the shade of a very thick tree-top the sun-flecks are circular like the sun; but during an eclipse they are crescentic, or even annular, according to the degree of obscuration. They are always images of the sun.

Property of a Lens.-Now a lens has the remarkable property of accomplishing both these apparently oppo

site ends, viz., brightness and distinctness at the same time. If an object, a c, be placed before a lens, L (Fig. 7), then all the rays diverging from any point, b, are

bent so as to come together again at the point b'. Of the divergent pencil, b L L, the central ray passes straight through without deviation; rays a little way from the central are bent a little; rays farther away are bent more and more according to their angle of divergence, so that they all meet at the same point, b'. Similarly all the rays proceeding from a, and falling on the lens, are brought to the same point, a', and from e to the point c', and so also for every intermediate point. Thus an image is formed which is both bright and very distinct if the receiving screen is suitably placed, i. e., at the exact place where the rays meet. The billions. of rays from millions of points of the surface of the object are, as it were, sifted out by the law of refraction, and each safely conveyed to its own point in the image; so that, for every radiant point of the object, there is a corresponding focal point in the image. But it is evident that the screen must be suitably placed, for, if it be placed too near, at S' S', the rays have not yet come together; if too far, at S" S", the rays have already met, crossed, and again diverged. In both cases the image will be blurred.

In all dioptric instruments images are formed in this way. It is in this way that images are formed in the eye. In Fig. 8 it is seen that the diverging pencils,

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 rèsemblances, 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