CHAPTER XI 

 THE EYE 



All kinds of organisms are affected by light. We have 

 seen that its influence upon the simpler forms, es- 

 pecially those not protected by pigment, is generally 

 destructive. In less intensity it acts as a stimulus, 

 modifying the behavior of the plants and animals upon 

 which it falls. Some cells retreat from it while green 

 plants grow toward its sources to utilize its energy in 

 chemical syntheses. Quite low in the scale we notice 

 animal types provided with what we call eye-spots, 

 particles of peculiar substance which we believe to be 

 more sensitive to the effects of light than the other 

 protoplasm of their cells. 



There is a great difference between being influenced 

 by light and being able to see. To see we must have 

 detailed pictures formed upon the retinas, and we must 

 have the requisite nervous and cerebral connections 

 to make possible the analysis and interpretation of these 

 pictures. If ground glass were kept before the eyes we 

 could estimate degrees of light and might notice the 

 coming and going of large shadows. It is probable 

 that the visual powers of many organisms, snails, for 

 example, are no better than this. We should be sub- 

 ject to the same limitations if the light fell directly 

 upon our retinas without passing through the optical 

 systems which refract and focus it. 



The human eye is a camera. It is a globe about 1 

 inch in diameter with a small area projecting in front 

 as though a second globe of less diameter were imbedded 

 in the larger one. The region which projects beyond 

 the regular curvature of the eyeball is exquisitely clear. 



149 



