THE SENSES 815 



have been formed. Some have supposed that the absorbed 

 light-waves are transformed into long heat-waves, and that 

 the endings of the optic nerve are thus excited by thermal 

 stimuli. This hypothesis has so little evidence in its favour 

 that it is perhaps an unjustifiable waste of time even to 

 mention it. It is ruled out of court by the mere fact that 

 the long radiations of the ultra red, filtered from luminous 

 rays by being passed through a solution of iodine, and 

 focussed on the eye by a lens 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 light undoubtedly causes (p. 647) 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, notwithstanding 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, since 

 the elaborate investigations of Kiihne, lost much of its 

 significance in this regard. But although the visual purple 

 has thus disappointed the hopes excited in sanguine minds, 

 and has not explained, or even lessened, the mystery of 

 vision, its discovery is in itself so interesting and so sugges- 



