THE SENSES 



151 



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 re- 

 ceives the light rays, and 

 retains the image for a 

 fraction of a second (Exp. 

 1 1 ). Hence the pictures 

 in a kinetoscope (Fig. 123) 

 appear as one moving pic- 

 ture. 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. 



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. Com- 

 pare the pictures in the figure. 



