FIG. 13.

spherule (Fig. 13). The surface of the cornea has the form of an ellipsoid of revolution about its major axis, and therefore doubtless contributes to the same effect. In looking at very near objects, the contraction of the pupil, also, by cutting off marginal rays, tends in the same direction. However the result may be accomplished, whether by one or by both methods, it is certain that in good eyes it is completely achieved, for the clearness of vision is wholly conditioned on the sharpness of the retinal image.

SECTION SHOWING THE
STRUCTURE OF THE
LENS.

It is probable that the peculiar structure of the crystalline lens described above has also another important use in the lower animals, if not in man. Dr. Ludimar Hermann has shown that, in a homogeneous lens, while the rays from radiants near the middle of the field of view, i. e., nearly directly in front, are brought to a perfect focus, the rays from radiants situated near the margins of the field of view, i. e., of very oblique pencils, are not brought to a focus. Therefore the picture formed by such a lens is distinct in the central parts, but very indistinct on the margins. Now, this defect of a homogeneous lens, Dr. Hermann shows, is entirely corrected by the peculiar structure of the crystalline; therefore this structure confers on the eye the capacity of seeing distinctly over a wide field, without changing the position of the point of sight. This capacity he calls periscopism. We will hereafter, however, give reasons showing that this property of the crystalline can be of little value to man.

5. Adjustment for Light.-The delicate work done by the camera and by the eye requires a proper regulation * " Archives des Sciences,” vol. lxiii, p. 66. 1875.

In

of the amount of light. In both, therefore, we want some contrivance by which, when the light is very intense, a large portion may be shut out, and when the light is feeble, a larger portion may be admitted. optical instruments this is done by means of diaphragms. In the camera we have brass caps with holes of various sizes, which may be changed and adapted to the intensity of the light. In the microscope we have a circular metallic plate, with holes of various sizes. By revolving this plate we bring a larger or a smaller hole in front of the lens.

In the eye the same end is reached, in a far more perfect manner, by means of the iris. The iris (Fig.

14, d) is an opaque circular disk, with a round hole, the pupil, in the middle. The circumference of the disk is immovably fixed to the sclerotic at its junction. with the cornea; but the margin of the circular hole, or pupil, is free to move. The disk itself is composed of two sets of contractile fibers, viz., the radiating and the

circular (Fig. 15). The radiating fibers converge from the outer margin of the iris as a fixed point, and take hold on the movable margin of the pupil, and, when they contract, pull open the pupil on every side, and thus enlarge it (Fig. 15, B). The circular fibers are concentric with the pupil, and are especially numerous and strong near the margin, forming there a band about one-twentieth of an inch wide. When they contract, they draw up the pupil, like a string about the mouth of a bag, and make it small (Fig. 15, A). We may regard the radiating fibers as elastic, and as contracting passively by elasticity when stretched; and the circular fibers as contracting actively under stimulus, like a muscle. Further, the circular fibers are in such sympathetic relation with the retina, that a stimulus of any kind, but especially its appropriate stimulus, light, applied to the latter, causes the former to contract, the extent of the contraction being of course in proportion to the intensity of the light. If, therefore, strong sunlight impresses the retina, the circular fibers immediately contract, the pupil becomes small, and a large portion of the light is shut out. When the light diminishes, as in twilight, the circular fibers relax, the previously stretched radiating fibers contract by elasticity, and enlarge the pupil. At night the pupil enlarges still more, in order to let in as much light as possible. Finally, if a solution of belladonna (which completely paralyzes the circular fibers) be dropped into the eye, the pupil enlarges so that the iris is reduced to a narrow dark ring.

Art, taking the hint from Nature, and striving to be not outdone, has recently constructed for the microscope a diaphragm somewhat on this plan. It is composed of many very thin metallic plates, partly covering each other, so arranged as to leave a polygonal hole in

the middle, and sliding over each other in such wise
that by turning a milled head in one direction they all
move toward the central point and diminish the open-
ing, while by turning in contrary direction they all
move away from the center and make the hole larger.
This is confessedly a beautiful contrivance, but how
inferior to the admirable work of Nature!

As already stated (page 37), contraction of the pupil
takes place not only under the stimulus of light, but
also in looking at very near objects. The reason of
this is, that correction of spherical aberration is thus
made more perfect.

* 6. Adjustment for Distance-Focal Adjustment. -We have seen that a lens, properly corrected for chromatism and aberration, makes a perfect image. But the plate or screen which receives the image and makes it visible must be placed exactly in the right place, i. e., in the focus; otherwise the image will be blurred. We reproduce here (Fig. 16) the diagram

on page 27, showing this. It is at once seen that, if
the receiving plate is too near the lens, i. e., at S' S',
the rays from any radiant of the object will not yet
have come together at a focal point. If the receiving
screen be too far from the lens, at S" S", then the rays
moving in straight lines will have already met, crossed,
and again spread out. It is evident that there is but, one

* Explain here how & focus is diff for divergs rays parallel rays.

place where the image is perfect, viz., at the focal points, SS. Now, if this place of the image were the same for all objects at all distances, it would be only necessary to find that place, and fix the receiving plate immovably there. But the place of the image formed by any lens changes with every change in the distance of the object. As the object in front approaches, the image on the other side recedes from the lens. As the object recedes, the image approaches the lens. Therefore there must be an adjustment of the instrument for the distance of the object.

There are only two possible ways in which this adjustment can be made: Either (1st), the lens remaining unchanged, the screen must advance or recede with the image; or (2d), the place of the screen remaining the same, the lens must be changed so as always to throw the image on the immovable screen. The first is the mode of adjustment used in the camera, the opera-glass, the field-glass, and the telescope; the second is the mode usually used in the microscope. In the camera, for example, when the object comes nearer, we draw out the tube so as to carry the ground-glass plate a little farther back; when the object recedes, we slide up the tube so as to bring the receiving plate nearer the lens. So in the opera-glass we elongate the tube for near objects, and shorten it for more distant. In the microscope, on the contrary, the image is usually thrown to the same place in the upper part of the tube. If, therefore, the object approaches nearer the lens (as it does in higher magnification), we change the lens so as to throw the image to the same place.

How is this managed in the eye? It was long believed that the adjustment was on the plan of the camera. Now, however, it is known that it is rather on