312 STEREOSCOPIC VISION. 



most common. This is a matter of importance among railroad 

 men and sailors, where it is necessary to distinguish red and 

 green signals. 



Stereoscopic Vision. In looking at an object with one 

 eye there is seen more to the side of that eye, while the other 

 eye sees more of the other side, considerable of the object 

 being seen with both eyes. The effects produced on the two 

 eyes are united, and so we better see objects as solids. This 

 is what is termed stereoscopic or Binocular Vision. 



Duration of Impressions of Light. Most boys have 

 amused themselves around a bonfire by whirling a stick with 

 a glowing coal on its end. The continuous circle of light thus 

 produced indicates that the impression of light remains for a 

 time, in this case until the stick completes the circle, giving 

 a continuous line of light. By Shaking the hand up and down 

 rapidly the same effect may be produced ; that is, we see the 

 hand in all positions at the same time. Or when riding in a 

 carriage the spokes of the wheels blur together because the 

 impression of each lingers till another has taken its place. 

 But if we shut the eyes quickly, we may keep distinct the 

 impression of the last positions, and so see them distinct from 

 each other. Better still, shut the eyes while looking at the 

 wheel, then open and shut them as quickly as possible. 



Again, if one looks at a bright lamp and then closes the 

 eyes, there may remain the same appearance as when we 

 looked at the object itself. This is called the Positive After- 

 image. Or sometimes, especially after looking long at a 

 bright light, we may, on closing the eyes or looking away, 

 see a dark spot of the same shape as the bright one we looked 

 at. This is called the Negative After-image. 



The Care of the Byes. 1. In reading we wish light from 

 the printed page. Hence we should avoid light entering the 

 eye from any other source at this time. While reading, then, 



