282 GENERAL SCIENCE 



image. The disturbance in the nerve caused by the 

 image is carried by the optic nerve to the brain, where it 

 is interpreted, and then we become conscious of the pres- 

 ence of an object in our field of vision. We see objects 

 because they reflect light into the eyes or because the 

 objects are self-luminous. Our eyes, of course, reflect light 

 so that another person can see them, but that part of the 

 eye just in front of the pupil does not reflect any light, 

 therefore the pupils of every person's eyes are black, and 

 we can only distinguish the pupil of the eye by means of 

 contrast. The iris, a muscle which regulates the size of 

 the pupil, having the color blue, gray, or brown, reflects 

 light as does the white part of the eyeball. 



The nerves of the retina are able to receive about ten 

 image impressions per second; that is, if ten objects were 

 shown to us in a second, we could see each one distinctly. 

 If less than ten are shown, we of course see them 

 separately and each one makes a distinct impression. 

 If more than ten objects are shown to us in a second, we 

 cannot distinguish one from the other and they appear as 

 a continuation of one object. If a stick with a spark of 

 fire on the end is whirled rapidly, we see a streak of fire 

 and not a spark in distinct successive positions. This is 

 because the stick gets around the circle and makes a 

 second impression or image on the retina before the im- 

 pression of the first image is gone. The separate spokes, 

 in a rotating wheel cannot be seen for the same reason. 

 The wheel of a rapidly moving automobile seems to be 

 solid and not made of spokes. Moving pictures are 

 operated on the same principle. A reel contains about 

 1,000 separate pictures an inch square. These are made 

 to pass the lens in the projection lantern at the rate of 

 fifteen or sixteen per second. Each picture shows the 



