204 THE POPULAR SCIENCE MONTHLY. 



is lost. Newton supposed that it was an impossibility to con- 

 struct a lens corrected for color which would magnify objects ; 

 but since the discovery (in 1753 and 1757) of different kinds of 



glass having the same refractive power 



CROWNJ3LASS 



but widely different dispersive powers, 



perfect lenses have been possible. 



In the human eye, a practically perfect 

 image, with no alteration in color, is pro- 



rio. 5. ACHROMATIC LENS. -i i 



duced by a mechanism which human inge- 

 nuity can not imitate. There is a slight error in the cornea, which 

 is corrected by an opposite error in the crystalline lens ; the iris 

 plays the part of the diaphragm of optical instruments and shuts 

 off the light from the borders of the crystalline lens, where the 

 error is greatest, particularly in near vision ; the curvatures of 

 the lens are not perfectly spherical, but are such that the form of 

 objects is not distorted ; and while such curvatures are theoretic- 

 ally calculable, their construction is practically impossible, as ex- 

 perience has shown ; different layers of the crystalline lens have 

 different dispersive powers; and thus a practically perfect image, 

 with no appreciable decomposition of white light, is formed on the 

 retina. 



Another wonderful thing about the eye, which adapts it most 

 beautifully to our requirements, is the division of the sensitive 

 parts of the retina into a very small area for distinct vision, 

 which we use for reading, for example, and a large surrounding 

 area in which vision is indistinct. If we saw with equal distinct- 

 ness with all parts of the retina, the vision of minute objects 

 would be confused and imperfect. As it is, the area of distinct 

 vision is very small, probably less than one thirty-sixth of an 

 inch in diameter. In this area, the distance between the separate 

 sensitive elements is not more than one thirty-five-hundredth of 

 an inch ; while, if we pass from this only eight degrees, the dis- 

 tance is increased a hundred times. Still, in looking at any one 

 object in the center of distinct vision, the imperfect forms of sur- 

 rounding objects are appreciated, warning us, perhaps, of the ap- 

 proach of danger. 



The mechanism of distinct and indistinct vision has been 

 understood only since 1876. The sensitive parts of the retina are 

 little rods and cones forming a layer by themselves. In 1876, 

 Boll discovered that in frogs kept in the dark the rods of the 

 retina were colored a dark purple ; but on exposure to light the 

 color faded, becoming first yellow and then white. Since that 

 time, physiologists have been carefully investigating visual pur- 

 ple and visual yellow. Just outside the layer of rods and cones are 

 the dark cells which render the greatest part of the interior of the 

 eye almost black. In the dark, these cells send little filaments be- 



