vessels and a loose connective tissue containing many dark brown or black pigment granules. The choroid absorbs superfluous light. Cats' eyes shine at night because this coat in their eyes reflects some light. The choroid separates from the sclerotic toward the front of the eye and forms the colored iris. The iris makes the eyes beautiful, and it also serves the useful purpose of regulating the amount of light. The hole in the iris is called the pupil (Exp. 15).
The third and innermost coat, the sensitive pinkish layer called the ret'in-a, is the most important and characteristic tissue in the eye. It receives the light rays, and retains the image for a fraction of a second (Exp. 11). Hence the pictures in a kinetoscope (Fig. 123) appear as one moving picture. The retina is made chiefly of the fibers of the optic nerve. This nerve contains about five hundred thousand fibers, and enters at the back of the ball. The spot where it enters contains no nerve endings and is not sensitive to light. It is called the blind spot. The spot where the light most often falls is most sensitive to light. It is the yellow spot (Fig. 122).
Test for the Blind Spot.—In this experiment shut the right eye and be careful not to let the left eye waver.
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Fig. 123.—Stroboscope, the original of the kinetoscope. The observer looks through the slits of a rapidly revolving disk and a new image falls on the retina before the last image has faded. Compare the pictures in the figure.
