THE SENSE OF SIGHT. 295 



Color Blindness. It is found that some persons can- 

 not distinguish certain colors. Blindness to red and green 

 are 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 more is seen 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 them- 

 selves 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 com- 

 pletes the circle, giving a continuous line of light. 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. 



After-Images. But if we shut the eyes quickly, we may keep dis- 

 tinct 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, espe- 

 cially 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 EYES. 



i. Objectionable Light. 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, do not face a window, another light, a mirror, 



