TKANSACTIONS OF SECTIOJf D. 739 



eacli fibre must be connected with several terminals, the number increasin|^ more 

 and more towards the periphery of the retina, where the power of perceiving a 

 double impression is far less than at the yellow spot. Preyer supposes that the 

 pairs of cones are in two sets mingled together ; the pairs of one set being for the 

 reception of red and green stimulation, the other for yellow and blue. The part 

 of Preyer's theory that appears to me attractive is that the photo-chemical chan"-es 

 induced iu the several terminals cause vibrations to be transmitted throun-h the 

 nerve whose frequency must be far less than that of the undulations of the ether 

 and their wave-length consequently^ far greater, but ' the ratios of the wave-lengths 

 of the ether and nerve vibrations probably remaining almost unchanged.' ' If 

 Preyer had only gone a step farther and supposed that different colour sensations 

 are producible in the same cells by difi'erent frequencies of vibrations entering the 

 sensorium through the same nerve fibres, I would gladly have assented to a theory 

 which would have been essentially a return to that of Newton : but my learned 

 friend still strives to account for different colour sensations on the doctrine of 

 specific activities, and to support the idea that different visual substances are 

 located in different optic terminals. He supposes that when the cones of the red- 

 green pair are simultaneously stimulated, vibrations of diiTerent frequencies are 

 transmitted to a ganglion cell in the retina, and thence to a fibre of the optic 

 nerve, but not simultaneously, lie does not entertain the idea that the two sets of 

 vibrations are compounded, and so transmitted by the fibre ; on the contrary, he 

 helieves that * one and the Same nerve fibre cannot be stimulated in a double 

 manner at the same moment,' and therefore, to explain the transmission of vibra- 

 tions of two frequencies by the same fibre, he supposes that there must be a rapidlv 

 alternating transmission of the two sets of vibrations. I feel unable to assent to 

 that idea, for it appears to me that if an undulatory theory is applicable to nerve 

 action, we must believe that simple pendular vibrations of different frequencies can 

 be compounded. Nor do I feel able to assent to Professor Preyer's theory of the 

 production of two qualities of sensation — say red and green — by vibrations trans- 

 mitted through the same fibre. IJeduced to the simplest expression, his theory is, 

 that the fibre eventually ends in two sensory cells of specifically diflPerent activities, 

 one producing a red. the other a green sensation ; that these two cells are affected 

 sympathetically by vibrations of different frequencies — the red cell by vibrations of 

 the frequency transmitted from the red cone, the green by those of the frequency 

 transmitted from the green cone. It seems to me far simpler to suppose that the 

 vibrations produced by chemical change in visual substances in the retina may be 

 transmitted either as simple or compound vibrations simultaneously by the same 

 nerve fibres, and produce different sensations in the same cells, according to 

 frequency and complexity of the incoming vibrations. 



It seems to me that the difficulty of this question is much increased by attempt- 

 ing to show that different cones are concerned in the reception of different colour 

 impressions. In support of such a view, the fact is adduced that, in birds and 

 some reptiles, each cone contains an oil globule of a red or orange colour iu some 

 cones, and of a yellow or 'greenish-yellow in others. The coloured spheroid is 

 placed in the outer part of the inner segment immediately internal to the outer 

 segment, and must exercise a selective absorption on the liglit passing to the outer 

 vsegment. The presence of these coloured spheroids is remarkable, but we know as 

 little regarding tlieir physiological significance as we do regarding that of the 

 yellow pigment in the internal layers of the retina at the yellow spot in 

 our own eyes, through which all the light must pass ere it can reach the 

 inner segments of the cones in that portion of the retina. All the cones 

 in the human eye, as in that of all mammals, are colourless, and it involves serious 

 difficulty if we suppose that different cones are affected by red, green, yellow, and 

 blue light. The image of a coloured star, small enough to fall on only one cone, 

 can Ije seen of a fixed and definite colour, that does not alter when the position of 

 the eye is changed, and the image shifted from one cone to another in the yellow 

 spot. That fact alone seems to me sufficient to show the necessity for supposing 

 that each cone is capable of stimulation by all visible undulations of light, and of 



' Preyer, ihid., p. 08. 



3 n 2 



