RETINAL CHANGES INVOLVED IN VISION 



629 



Its importance will be at once appreciated if one eye be closed 

 and the stimulation of the peripheral parts of the retina in the other 

 eye be excluded by allowing this eye only to look through a tube. 

 Although we can then see the objects to which we direct our gaze 

 perfectly distinctly, we find that on trying to move towards any given 

 object our movements are uncertain and misdirected. 



CHEMICAL AND PHYSICAL CHANGES IN THE RETINA 

 When light falls on the retina chemical and physical changes 

 take place ; these either originate or accompany the transmutation 



xn 



XI 



n 



vm 



IV 



vn 



FIG. 289. Perimeter chart showing the field of vision in a normal (right) eye. 



of the ether vibrations into the nerve impulses, which ascend the optic 

 nerve. If a frog that has been in the dark for some time be killed, an 

 eye taken out, bisected, and the retina removed and examined by a 

 weak light, it will be found that this latter has a purplish-red colour. 

 On microscopical examination this colour is seen to be confined to the 

 outer limbs of the rods. After a very short exposure to diffuse day- 

 light the colour disappears. The colouring-matter (rhodopsin) may 

 be dissolved out by means of a solution of bile salts. The purple-red 

 solution thus formed also bleaches rapidly on exposure to light. By 

 means of this rhodopsin photographs or ' optograms ' of external 

 objects may be taken on the retina. The rabbit's eye is cut out and 

 placed in front of a window. After some time the eye is bisected and 

 plunged into a 4 per cent, solution of alum, which partially fixes the 



