THE SENSES 933 



and men, in whose eyes pigment is absent, can see. In man 

 and most mammals there are cones, but no rods in the yellow 

 spot and fovea centralis ; the relative proportion of rods in- 

 creases as we pass out from the fovea towards the ora serrata. 

 But this does not enable us to analyze the bacillary layer into 

 sensitive cones and non-sensitive rods, for on the rim of the 

 retina, which is still sensitive to light, there are only rods ; in 

 the bat and mole there are said to be no cones even in the yellow 

 spot, in the rabbit very few. Reptiles possess only cones over 

 the whole retinal surface, and birds, true to their reptilian 

 affinities, have everywhere more cones than rods, as have also 

 fishes. 



One of the difficulties in the way of understanding how a 

 ray of light can set up an excitation in a rod or cone is the 

 transparency of these structures. An absolutely transparent 

 substance that is, a substance which would allow light to 

 traverse it without the least absorption would, after the 

 passage of a ray, remain in precisely the same state as before ; 

 its condition could not be altered by the passage of the light 

 unless some of the energy of the ethereal vibrations was trans- 

 ferred to it. But an absolutely transparent body does not 

 exist in Nature ; and it is not necessary to suppose that all the 

 energy required to stimulate the end-organs of the optic nerve 

 comes from the luminous vibrations. These may, and probably 

 do, act by setting free energy stored up in the 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 trans- 

 verse 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 formulated. 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 



