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TEXT-BOOK OF PHYSIOLOGY. 



of the object from the lens. The presence of the visual purple in the 

 rods of the retina capable of being altered by light makes the com- 

 parison still more striking. 



Kiihne even succeeded in obtaining a fixed image or an optogram 

 of an external object in a manner similar to that by which an image 

 is fixed on the sensitive plate of a camera. An animal is kept in 

 the dark for about ten minutes in order to permit the retinal pigment 

 to be completely regenerated. The animal, with the eyes covered,, 

 is then brought into a room with a single window. After the head 

 is steadily directed to the window, the eye is exposed for several 

 minutes. The eyes are again covered, the animal killed, and the 

 eyes removed by the light of a sodium flame. The retina is then 



FIG. 302. SECTION OF THE RETINA OF A FROG. A. In darkness. B. In light. 

 (After Van Genderen Stort, from T scheming' s "Physiologic Optics."} 



placed in a 4 per cent, solution of alum. In a short time the image 

 of the window, the optogram, will be fixed (Fig. 303). That portion 

 of the retina corresponding to the image is quite bleached in appear- 

 ance from the action of the light on the pigment. During life the 

 regeneration of the visual purple must take place with extreme 

 rapidity. It is believed to be derived from a pigment secreted by 

 the layer of pigment cells. 



Binocular Vision. Though two images are formed, one on each 

 retina, when the eyes are directed to a given object, there results but 

 one sensation. If the direction of either visual axis be changed by 

 pressure on the eyeball, there arise two sensations, and the object 

 appears to be doubled. The reason assigned for this, in the first 



