540 STUDIES IN ADVANCED PHYSIOLOGY. 



of the refraction of light is based the use of spectacles, and 

 even the use of the eye itself, this containing within itself 

 several refracting media which obey in every way the physi- 

 cal laws that govern light refraction. 



8. The Formation of Images by Lenses. By means of 

 a good lens or a system of lenses there may be produced 

 an image of an object placed in front of the lens. The 

 microscope, the telescope and the photograph camera at- 

 test this fact. In the camera a sensitive plate is put back 

 of the lens in such a position that the image produced by 

 the lens falls directly on it, and there in chemical ways 

 leaves its impress on the sensitive plate. There is in the 

 eye an exactly analogous result, the eye being in practically 

 every particular a high grade camera in which the sensitive 

 plate is replaced by the still more sensitive retina, and in 

 order to perceive objects clearly the image produced by the 

 lens and humors of the eye must fall directly upon the re- 

 tina. The point under explanation here is to show under 

 what circumstances an image arises. An image results 

 when a number of rays of light emanating from the same 

 point are so bent by a lens as to meet again at a certain 

 point back of the lens. As the rays of light from any point 

 on a luminous body radiate in every direction; that is, 

 diverge more and more, it is evident that they would never 

 meet unless these divergent rays should be bent together 

 by some refracting medium; hence no image is possible 

 when the lens is removed. The function of the lens is to 

 collect this divergent pencil of rays from the point in qiies- 

 tion, and bend it together to meet in a corresponding point 

 back of the lens. 



If at this second point a screen be held there appears an 

 image the exact counterpart of the object. But any object 

 consists of but an infinite number of individual points, and 

 so it is the function of the lens to take the divergent rays 

 from each of these infinite points and converge them to a 

 corresponding number of infinite points behind the lens. 



