738 BEroHT— 1892. 



It is difficult to see how these results are to be harmonised with the conclusions 

 arrived at by the Committee of the Iioyal Society. 



Facts such as these are regarded by some as lendinp- support to the theory of 

 colour sense proposed by Professor Herinpr, of Prague.* He supposes that the 

 diversity of our visual perceptions arises from six fundamental sensations consti- 

 tuting three pairs — white and black, red and green, yellow and blue. The three 

 pairs of sensations are supposed to arise from chemical changes in three visual 

 substances not confined to the retina, but contained also in the optic nerve and in 

 the vision centre.- He imagines that a sense of white results from decomposition 

 induced in a special Tisual substance by all visible rays, and that the restitution of 

 the same substance produces a sense of black. The sensations of the red and 

 green pair are supposed to arise, the one from decomposition, the other from 

 restitution of a second substance ; while yellow and blue are supposed to result 

 from decomposition and restitution of a thii'd substance. From our knowledge of 

 photo-chemical processes we can readily suppose that light induces chemical change 

 in the visual apparatus ; but that the wave-lengths in the red and yellow parts of 

 the spectrum induce decomposition, while the wave-lengths in the green and blue 

 induce restitution of substances, it is difficult to believe. How such a visual 

 mechanism could work it would be difficult to comprehend ; for example, if we 

 look at a bright red light for a few moments and then close our eyes, the sensatiou 

 remains for a time, but changes from red to green and then slowly fades away. 

 According to Hering's theory, the green after-sensation results from the restitution 

 of a substance decomposed by the red light. But if we reverse the experiment by 

 looking at a bright green light and then closing our eyes, the after-sensation 

 changes to red. The theory in question would require us to suppose that the green 

 light builds up a visual substance which spontaneousl}^ decomposes when the eyes 

 are closed, and so produces the red after-image. I'coufess that such an hypothesis 

 seems to me incredible. Another remarkable feature of Hering's theory is that 

 colours termed complementary ought to be termed nntagonistic,^ because they are 

 capable of producing a colourless sensation when mingled in due proportions. If 

 the complementary colours yellow and blue could, when mixed, produce black, 

 they might well be named 'antagonistic ' ; but since their combined effect is a sense 

 of whiteness, and since the addition of them to white light increases its luminosity, 

 it seems very difficult to comprehend on what ground the term antar/onistic should 

 be substituted for complementary. I confess I am quite imable to follow Heriug 

 when he supposes that three pairs of mutually antagonistic chemical processes are 

 produced in the retina when white light falls on it, that these processes are all 

 continued on through the optic nerve into the vision centre, and there give rise to 

 our different light and colour sensations. 



In 1881 Professor Preyer contributed an important paper on colour sense,'' in 

 which he supported Young's theory of primary colour sensations, but assuming 

 their number to be four instead of three. Believing with Schultze that the cones 

 are the only terminals in the retina concerned in colour sense, Preyer supposes 

 there are four sets of cones containing different photo-chemical substances which 

 are severally affected by undulations in the red, yellow, green, and blue parts of 

 the spectrum. But since it has been calculated that the cones are thrice as 

 numerous as the fibres of the optic nerve, Preyer has endeavoured to bring the 

 Helmholtz amplification of Young's theory within the range of anatomical possi- 

 bility by supposing that the four sets of cones are arranged in pairs, each pair 

 being connected with one nerve fibre. This hypothesis, however, could not meet 

 the case, oven if cones were the only visual terminals. The rods are far more 

 numerous than the cones ; therefore, although it is probable that in the yellow spot 

 each nerve fibre is connected with only one or two cones, elsewhere in the retina 



' E. Hering, Zur Lelire vovi Llchtsinne, 2nd ed. Vienna, 1878. 

 - Hering, ihid., p. 75. 



* Hering, -ihid., p. ]21. 



* W. Preyer, 'Uber den Farben und Temperatur Sinn, mit besondercr Kiick- 

 sicht auf Farbenblindheit,' Archir fur Phyftologie, 1881, vol. xxv. p. 31. 



