§ 3. Structure of the Eye.—For anatomical detail we must refer to the textbooks of physiology. The eye as a whole is analogous to a photographic apparatus. "In it a camera or dark chamber of notable size exists similar to that which a photographer uses, having a lens in the forepart, and a sensitive curtain at the back.....When the photographer looks in at the back of his camera, he sees on the ground glass plate the image depicted which he wishes to photograph, placed upside down, but faithfully delineated in all its colours; and such an inverted landscape is formed in like manner in the back part of each of our eyeballs. And as the photographer adjusts the focus of his instrument by altering the position of the lens, screwing it nearer or further from the screen, so we adjust the focus of our eye instinctively according to the distance of the object looked at, not indeed by changing the position of the lens but by altering its form so as to make it stronger or weaker as required."*

The sensitive curtain is called the retina; in its centre there is a circular depression called the fovea centralis. This pit and its immediate margin is also called the yellow spot, from its colour. In ordinary light this is in all respects by far the most discriminative part of the retina, and it alone gives distinct vision of an object. Near it, on the nasal side, the optic nerve enters the eye, and this point, not being sensitive to light, is called the blind spot.

The retina is an expansion of the optic nerve. Its essential constituents are certain minute cells of two kinds, called respectively rods and cones. The yellow spot consists mainly of cones closely packed together. In other parts the rods predominate. The number of cones decreases from the yellow spot to the margin of the retina.