The human eye is a nearly spherical organ, capable of tolerably free movements of rotation in its socket. What we may call the outer case, which is white and opaque elsewhere, is quite transparent in front. Through this transparent window may be seen the coloured iris, in the centre of which is a circular aperture, the pupil. The size of the pupil changes with the amount of light--it dilates or contracts, according as the light is less or more intense. Just behind it, and still in the front part of the eye, is the transparent lens, the convexity of the anterior surface of which can be altered in the accommodation of the organ for near or far vision. The space between the lens and iris and the corneal window of the eye is filled with a watery fluid. Behind the lens there is a transparent, semi-fluid, jelly-like material, filling the rest of the chamber of the eye. At the back of the eye is spread out the sensitive membrane--the retina. The structure of this membrane is very complicated, and cannot be described here. It is, however, indicated in Fig. 32. For our present purpose it is sufficient to note that here are the end-organs of the optic nerve; that these consist of a number of delicate rods and cones; and that these rods and cones do not face in the direction from which the light comes, but face towards the back of the eyeball, where a pigmented substance is developed. The rays of light are thus focussed through the retina on to this pigmented substance; the ends of the rods and cones are stimulated; and the stimulation is handed on, augmented in certain intermediate ganglia, to the delicate transparent nerve-fibres in the front of the retina. These collect to a certain spot, where they pass through the retina to form the optic nerve. Where they pass through the retina there can, of course, be no rods and cones. And in this spot there is no power of vision. It is the blind spot. The reality of its existence can easily be proved. Make a dot on a piece of writing-paper, and about three inches to the left of it place a threepenny or sixpenny bit. Close the right eye, and look with the left eye at the dot. The sixpenny bit will also be seen, but not distinctly. Keep the eye fixed on the dot, and move the head slowly away from the paper. At a distance of about ten inches the coin will completely disappear from view. Its image then falls on the blind spot.