740 REPOHT— 1892. 



transmitting sucli nerve vibrations as are capable of inducing all the colour 

 sensations. 



It further appears to me that the difticulty of the case is increased hy attempt- 

 ing to show that cones are the only optic terminals concerned in colour impressions, 

 while the rods minister only to a sense of light. It is true that cones are relatively 

 few in number, or may indeed be absent from the retina of nocturnal animals, such 

 as night-ilying owls and bats, but that cannot be regarded as proof that the rods 

 are not concerned in colour sense. Rods are much more numerous than cones 

 everywhere in the human retina, save at the yellow spot. Throughout the greater 

 part of the retina the cones occur singly, surrounded by numerous rods. Yet when 

 the image of a coloured .star, .small enough for the area of a single cone, is made to 

 fall on parts of the retina peripheral to the yellow spot, its colour does not dis- 

 appear, and reappear when the eye is moved, as would inevitably happen if the 

 rods were not terminals concerned, as well as the cones, in colour sense. 



It must be admitted that the production of nerve impulses within the terminals 

 in the retina is almost as obscure as ever. It is still the old question, Does light 

 stimulate the optic terminals by inducing vibration, or by setting up chemical 

 change ? Whichever view we adopt, it seems to me necessary to suppose that all 

 the processes for the production of nerve impulses can take place in one and the 

 same visual cell, and are transmitted to the brain through the same nerve fibre. 



I referred to the sense of smell because it seems to me that we cannot in that 

 case escape from the conclusion that the different sensations arise from different 

 molecular stimulations of the same olfactory terminals. 



From Lippmann's recent researches on the photography of colour ' it appears 

 that f 11 parts of the spectrum can now be photographed on films of albumino- 

 bromide of silver to which two aniline substances, azaline and cyanine, have been 

 added. It seems, therefore, reasonable to suppose that a relatively small number 

 of substances could enable all the rays of the visible spectrum to aft'ect the retina. 

 Helmholtz believes that three visual substances would sufiice ; but if the primary 

 sensations are to be regarded as four — red, green, yellow, and blue — at least four 

 visual substances appear to be necessary ; and I think we must assume that all of 

 them are to be found in the same visual cell in the retina, and that the nerve 

 impulses which their decompositions give lise to are all transmitted through the 

 same optic fibres to the brain cells, there to produce a sense of uncoloured or 

 coloured light. Evidently such an hypothesis is not altogether novel ; it is essen- 

 tially a return to that long ago suggested by Newton. The only ditt'erence is that 

 light is supposed to induce photo-chemical changes in the retina, as Von Helmholtz 

 suggested, instead of mere mechanical vibration, as Newton supposed. But if in 

 the sense of smell nerve undulations are induced by mechanical vibrations of mole- 

 cules acting on delicate hairs at the ends of cells, is it, after all, so very unreasonable 

 to suppose that within each visual cell there may be different kinds of molecules that 

 vibrate in different modes when excited by ether waves ? Four or five sets of 

 such molecules in each terminal element in the retina would probably be sufficient to 

 project successively or simultaneously special forms of undulations through the optic 

 nerve, to induce colour sensations differing according to the wave form of the incoming 

 nerve undulation. 



The photo-chemical hypothesis has much in its favour. We know how 

 rapidly light can induce chemical change in photographic films, and Ave know 

 that light induces chemical change in the vision-purple in the outer segments of 

 the rod cells in the retina. The fact that the cones contain no vision-purple 

 is no argument against the theory, for the inner segment of both rod and cone 

 is by many regarded as the true nerve terminal, and there is no vision-purple 

 in either of them. The visual substances in the cones, at all events, are colour- 

 less, and the existence of them as substances capable of producing nerve impulses 

 hy chemical decomposition is as yet only a speculation awaiting proof. The 

 fatigue of the retina produced by bright light is best explained on a chemical 



' G. Lippmann, ' On the Photography of Colour,' Comptes Rcndus, 1892, tome csir. 

 p. 9G1. 



