ioo6 THE SENSES 



retina, just as the touch of a child's hand could be made to fire a 

 mine, or launch a ship, or flood a province. Some have looked upon 

 the transverse lamellae into which the outer members of the rods 

 and cones can be made to split as an arrangement for reflecting 

 back the light to the inner members, and have compared them 

 to a pile of plates of glass, which, transparent as it is, is a most 

 efficient reflector. It is even possible, although here we are already 

 treading the thin air of pure speculation, that the light may be 

 polarized in the process of reflection, and that the rods and cones 

 may be less transparent to light polarized in certain planes than to 

 unpolarized light. 



As to the nature of the transformation undergone by the ethereal 

 vibrations in the rods and cones, various theories have been formu- 

 lated. 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 the passage of a voltaic current 

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

 doubtedly, causes (p. 811) 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 proposi- 

 tion 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 lost its significance in this 

 regard. But although the visual purple is not a photo-chemical 



