This faculty of accommodation in the eye is thus demonstrated: we place two pins, one in front of the other, one eye only being open; we first look at the nearest pin, which appears confused if it is near the eye, but by an effort of will the image becomes clear. If, while preserving the clearness of the image, we then carry our attention to the second pin, we find that it, too, presents a confused appearance. If we make an effort to distinguish the contour of the second pin, we at last succeed, and the first once more appears ill-defined. It is only since the experiments of M. Cramer and M. Helmholtz that the explanation of this phenomenon could have been given. M. Cramer has succeeded in determining on the living eye the curved ray of the cornea, and of the two surfaces of the crystalline lens. In so doing he followed Samson’s method, and observed the images thrown by a luminous object, whose rays strike the different refracting surfaces of the eye. A candle, L (fig. 98), is placed before the eye, O, and throws as in a convex mirror a straight image of the flame, A (fig. 99). The other portion of the light, which has penetrated the pupil, falls on the crystalline lens, and produces likewise a second straight image, B. Then the light refracted by the lens reaches the posterior surface; a portion is reflected on a concave mirror, and gives the inverted image, C, very small and brilliant. M. Cramer observed it through a microscope, and studied the variations in the size of images when the eye passed from the observation of adjacent to distant objects. He stated:— 1. That the image, A, formed on the surface of the cornea, remains the same size in both cases; the form of the cornea therefore remains unaltered. [Illustration: Fig. 98.—M. Cramer’s experiment.] [Illustration: Fig. 99.—Images in the eye.] 2. That the image, B, formed on the upper surface of the lens, diminishes in proportion as the eye is nearer the object; the surface therefore becoming more and more convex, as the focal distance diminishes—a result indicated by the theory that it is possible in the vision of near objects to receive the image on the retina. 3. That the third image, C, produced on the posterior surface of the lens, remains nearly invariable.