arranged as to leave a polygonal or nearly circular 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 opening, while by turning in the 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!

FIG. 16.

But contraction of the pupil takes place not only under the stimulus of light, but also in looking at very near objects. The purpose of this as already stated on page 31, is, that correction of spherical aberration is thus made more perfect.

d

d

с

Experiment.-An interesting and at the same time amusing experiment illustrating this point may be made thus: Stand in a darkish room. Cover the left eye with the hand. Gaze with the right eye at a distant point, say the wall, d. The optic axes are now nearly parallel, d R, d L. Tell a friend to observe the pupil of the open eye. It is probably greatly enlarged. Now, without changing at all the direction of the line of sight or position of the eye, run the point of sight, from d, through c, α and b, up to a, within three inches of the eye. The pupil is seen to contract to the size of a pinhole. You seem to have exercised voluntary control over the muscles of the iris. But not so. The contraction is purely consensual, with strong convergence of the optic axes. If the other eye were open, it would be

R

seen to turn strongly inward toward the nose to look

at a.

6. Adjustment for Distance-Focal Adjustment Accommodation.-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. 17) the diagram on page 21, 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 place where the image is perfect, viz., at the focal points, S S. 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. There

fore 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 (1), the lens remaining unchanged, the screen must advance or recede with the image, or (2), 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 operaglass, 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 the plan of the microscope. It was formerly thought that, in looking at a near object, the straight muscles, acting all together, squeezed the eye about the equatorial belt, and increased its axial diameter-in other words, made it egg-shaped-and thus carried the retinal screen farther back from the lens. But now it is known that the retinal screen remains immovable, and the lens changes its form so as to throw the image to the same place.

Experiment.-This is proved in the following manner: A person is chosen with good, normal young eyes. The experimenter stands in a dark room, in front of the patient, A, with a lighted candle in his hand, a little to one side, as in Fig. 18, C, while his own point

FIG. 18.

B

n

A, eye observed; B. eye of observer; c, section of candle-flame; f, a distant point of sight, and n a near point of sight. (After Helmholtz.)

FIG. 19.

of observation is on the other side, B. If the observer now looks carefully, he will see in the eye of the patient three images of the candle-flame: first, one reflected from the surface of the cornea, which is by far the brightest (Fig. 19, a); second, one from the anterior surface of the crystalline, much fainter (Fig. 19, b); third, one from the posterior surface of the crystalline, the faintest of all, and very small (c). Of course there is a fourth image, viz., that on the retina and which determines sight. The three images here spoken of are by reflection and have nothing to do with sight. The retinal image is by refraction, and we are not here concerned with it. Further, it will be observed that the first and second are erect images, because reflected from a convex surface, while the third is inverted, because reflected from a concave surface. Now, directing the patient to

gaze on vacancy, or a distant point, f, Fig. 18, we observe carefully the position and size of these several images. Then, if by request of the observer the patient transfers the point of sight to a very near point, n, without changing the direction, we observe that the images a and c do not change, but the image b changes its position and grows smaller. This image is reflected from the anterior surface of the crystalline. The anterior surface of the crystalline, therefore, changes its form.

FIG. 20.

a

F N

F, lens adjusted to distant objects; N, to near objects; a, aqueous humor; d, ciliary muscle; e, ciliary process.

Again, the nature of the change of the image, viz., that it becomes smaller, shows that this anterior surface becomes more convex. By careful examination the iris, too, may be seen to protrude a little in the middle. Evidently, therefore, in adjusting the eye to very near objects, the crystalline becomes thicker in the middle, and pushes the pupil a little forward. In the accompanying diagram, Fig. 20, the crystalline lens is divided by a plane through the center. The right side, N, is adapted to near objects; the left, F, to distant objects.

Theory of Adjustment.—It is certain that in adjusting the eye for looking at very near objects, the lens becomes more convex. But the question, "How is this done?" is more difficult to answer. Helmholtz thinks it is done in the following manner : *

"Optique Physiologique," p. 150.