582 



THE NERVOUS SYSTEM. 



siderable time, is then exposed so as to receive the image of 

 a window upon the retina for a time varying from several seconds 

 to several minutes, according to the intensity of the light, and 

 the retina is then removed and inspected in a red light, the 

 image of the window will be seen in it. Such an image is an 

 optogram, and is due to the action of the light on the visual purple, 

 bleaching it in some places, and but little affecting it in others. 

 This image may be preserved, or, as photographers say, " fixed," 

 by putting the retina in a 4 per cent, solution of alum. 



While it might at first seem as if these changes in the rhodopsin 

 explained what actually took place in the eye when the waves of 

 the luminiferous ether reached the retina and produced the sensa- 



FIG. 360. Model to illustrate astigmatism. 



tion of light, still the absence of this coloring-matter from the 

 cones, which exist without the rods in the fovea centralis, where 

 sight is most acute, would alone be sufficient proof that the visual 

 purple is not essential to vision. Some animals possessing sight 

 have no visual purple even in the rods. 



Engelmann has described a shortening and a thickening of the 

 cones of frogs and fishes under the stimulation of light, and a 

 lengthening in the absence of light, but as to the connection be- 

 tween these changes in the cones and sight, nothing is known. 



The eye is able not only to see objects, but to take cognizance 

 of certain facts in connection with them, such as their form, size, 

 distance, and color. 



