934 A MANUAL OF PHYSIOLOGY 



of rock-salt, produce not the slightest sensation of light, although 

 they are by no means all absorbed in their passage through the 

 dioptric media. Again, it has been suggested that the energy 

 of the waves of light is first transformed into electrical energy, 

 and that the visual stimulus is really electrical. In support of 

 this view it has been urged that the passage of a voltaic current 

 through the eye causes sensations of light and that light, un- 

 doubtedly, causes (p. 735) an electrical change in the retina 

 and optic nerve. But, as has more than once been pointed out, 

 an electrical change is the token and accompaniment of the 

 activity of the excitable tissues in general ; and all that the 

 currents of action of the retina show is that light excites the 

 retina a proposition which nobody who can see requires an 

 objective proof of, and which does not carry us very far towards 

 the solution of the problem how that excitation is brought 

 about. Then there is the photo-mechanical theory, according 

 to which the pigmented epithelial cells of the retina, altering 

 their shape and volume under the stimulus of light, press upon 

 the rods and cones, and thus mechanically stimulate them. 

 Lastly, there is the photo-chemical theory, which supposes that 

 some chemical change produced in the rods and cones under the 

 influence of light sets up impulses in them which ascend the 

 optic nerve. This is the most probable of all the theories, not- 

 withstanding the fact that the discovery by Boll of the famous 

 visual purple or rhodopsin, which at first seemed likely to place 

 it upon a sure foundation, has lost its significance in this regard. 

 But although the visual purple is not a photo-chemical substance 

 through which the retinal elements are excited by luminous 

 stimuli, it seems to fulfil an important function in adapting 

 the retina i.e., rendering it more sensitive for vision in dim 

 light. In any case, its discovery is in itself so interesting and 

 so suggestive as a basis for future work, that a short account of 

 the properties of the substance cannot be omitted here. 



Visual Purple. If the eye of a frog or rabbit, which has been kept 

 in the dark, be cut out in a dimly-lighted chamber or in a chamber 

 illuminated only by red light, and the retina removed, it is seen, 

 when viewed in ordinary light, to be of a beautiful red or purple 

 colour. Exposed to bright light, the colour soon fades, passing 

 through red and orange to yellow, and then disappearing altogether. 

 The yellow colour is due to the formation of another pigment, visual 

 yellow ; the preceding stages are due to the intermixture of this 

 visual yellow with the unchanged visual purple in different propor- 

 tions. With the microscope it may be seen that the pigment is 

 entirely confined to the outer segment of the rods, where it exists in 

 most vertebrate animals. It may be extracted by a watery solution 

 of bile-salts, and the properties of the pigment in solution are very 

 much the same as its properties in situ ; light bleaches the solution 

 as it does the retina. Examined with the spectroscope, the solution 



