538 STUDIES IN ADVANCED PHYSIOLOGY. 



green, blue, violet, and even some yellow are absorbed, and 

 so only the red light remains to be reflected. A rather far- 

 fetched analogy might be found in throwing against a wall 

 a handful of different objects, nearly all of them very sticky 

 indeed, but mixed with these several elastic rubber balls. 

 If such a handful of objects were hurled against a wall, not 

 all of them would be reflected. Most of the sticky objects 

 would remain attached to the wall, and possibly the rubber 

 balls alone be reflected. So in the sunlight, which is a 

 mixture of the fundamental colors, practically all of the 

 colors are absorbed by the ribbon, while the red, like the 

 rubber ball in the analogy, is reflected, and as we judge of 

 the color of an object by the light which comes from it, we 

 designate it as red. As a matter of fact, however, the color 

 of an object by reflected light is always the color which it 

 has rejected. 



7. The Refraction of Light and the Property of Lenses. 

 Light passes in straight lines through a medium of uniform 

 density. If, however, it passes from a less dense to a more 

 dense medium it is bent from its course. Such a bending 

 of rays of light is called refraction, and upon this property 

 of light depends the use of lenses. A lens is nothing more 

 than a transparent medium of greater density than the air, 

 and so will bend the rays of light from their original direc- 

 tion. It would be entirely out of place in this discussion to 

 go into detail in the matter of refraction, and it will suffice 

 to call attention to the ordinary kinds of lenses and the 

 manner in which they refract the light. 



Convex lenses, either convex on both sides (double con- 

 vex lens) or plain on one side (single convex lens) bend 

 the rays of light together, and if the lens be a fairly good 

 one, all the rays in a beam of light may be finally bent so 

 as to meet at a certain point back of the lens known as the 

 focus. The amount of the bending of the rays will depend, 

 other things being equal, upon the convexity of the lens, 

 being greater, the greater the convexity. Illustrations of 

 such results are familiar to ever one in the ordinary burning 



