THE SENSES 



779 



sented as being totally absorbed before passing through the substance. 

 Some of the green rays are reflected, some transmitted, some absorbed. 

 The red rays are supposed to be mostly reflected and trans- 

 mitted, only to a slight extent absorbed. The colour of such a 

 substance, both when looked at and when looked through, would 

 therefore be that due to a mixture of red light with a smaller 

 quantity of green. Then there is another class of substances which 

 owe their colour to selective reflection. Certain rays only are reflected 

 from their surface, and the light transmitted through a thin layer is 

 complementary to the reflected light ; that is, the reflected and trans- 

 mitted rays together would make up white light. These bodies have 



FIG. 286. DIAGRAMMATIC HORIZONTAL SECTION OF THE LEFT EYB. 



what is called ' surface colour] and include metals, various aniline 

 dyes, and other substances. 



Comparative. Many invertebrate animals possess rudimentary 

 sense-organs, by means of which they may receive certain luminous 

 impressions. It is true that the mere sensation of light is not in 

 itself sufficient for the exact appreciation of the form and situation of 

 surrounding objects. But even the closure of the eyelids does not 

 prevent a person of normal eyesight from distinguishing differences 

 in the intensity of illumination. And it is possible that many of the 

 humbler animals may, through the pigment spots which are often 

 called eyes, or perhaps, as in the earthworm, by means of end- 



