VISION 



973 



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 com- 

 plementary to the reflected light that is, the reflected and transmitted 

 rays together would make up white light. These bodies have 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 impres- 

 sions. 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. 



Cornea 



flfuscle 



-Selenitic 



-Retina 



M , u Optic Nerve, 



Fbvea Centmlis 



Fig- 397- Diagrammatic Horizontal Section of the Left Eye. 



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 earth- 

 worm, by means of end-organs more generally diffused in the skin, 

 attain to some such dim consciousness of light and shadow as will enable 

 them to avoid an obstacle or an enemy, to seek the sunny side of a 

 boulder or the obscurity of an overhanging ledge of rock. But the 

 indispensable condition of distinct vision is that an image of each part 

 of an object should be formed upon a separate portion of the receiving 

 or sensitive surface. This condition is, to a certain exentt, fulfilled by 

 the compound eyes of some of the higher invertebrates (insects, e.g.}. 

 Here rays from one point of the object pass through one of the funnel- 

 shaped elements of the compound eye, and rays from another point 

 through another. Rays striking obliquely on the facets are stopped 

 by the opaque partitions between them. In the Cephalopods we find 



