TRANSACTIONS OF SECTION I). 741 



theory, tut it could also be explained on a mechanical theory, for we must remem- 

 ber tiiat, even if the nerve impulses produced in the visual cells were merely a 

 translation of the energy of light into vibration of nerve molecules, the nerve 

 impulse has to pass through layers of ganglionic cells before reaching the fibres of 

 the optic nerve, and in these cells it probably always induces chemical change. 

 The phenomena of partial colour-blindness could be explained on a photo-chemical 

 theory by supposing that it arises from the absence of the substances required to 

 produce the wave forms necessary for the colour sensation which is defective, but 

 the total colour-blindness at the anterior part of the retina is evidently a difficulty. 

 How could we have a sense of light irom that portion of the retina if all the visual 

 substances are absent ? That is one of the reasons why Ilering supposed that a 

 special visual substance is present everywhere in the retina, which by decomposition 

 gives rise to a sense of light as distinguished from colour. But even on the hypo- 

 thesis we are pursuing, it is not necessary to suppose that all visual substance is 

 absent, for the colour-blindness in the front of the retina could be explained by 

 i^upposing that colour perception has not been developed in the corresponding 

 portion of the vision centre, and consequently all nerve impulses coming from that 

 part of the retina produce scarcely anything more than a sense of light. 



If the photo-chemical theory is entertained, it seems necessary to suppose that 

 there is some singular relation between the pairs of substances which respectively 

 give rise to red and green, and yellow and blue, seeing that both members of a 

 pair frequenth', if not always, fail together. 



It seems to me that the great difficulty arises when we consider the puzzling 

 phenomena of contrast. If light of a particular wave length decomposes a special 

 substance, and gives rise to, say, a sense of red, why does the complementary 

 bluish-green sensation appear in the remainder of the vision centre ? If the induced 

 colour were a pure green, one might attempt to explain it by supposing that a 

 sympathetic change had been induced in a substance closely related to that suffer- 

 ing decomposition by the objective light, but no such simple explanation is 

 admissible ; the complementary contrast of red is not green, but a mixture of 

 green and blue. The inadmissibility of such an explanation becomes still more 

 apparent if we take pure green as the inducing colour— the complementarj^ con- 

 trast that appears is purple, which involves a blue or violet, as well as a red 

 sensation. It matters not what inducing colour sensation we employ, the induced 

 contrast is always the complementary required to produce a sense of white. 

 George Wilson ' long ago suggested that simultaneous contrast probably arises 

 from a * polar manifestation of force ' ; indeed, he regarded it as a ' true, though 

 unrecognised, manifestation of polarity.' It is enough to mention that interesting 

 suggestion, but I must not pursue it, for we are dealing with a problem that has as 

 yet baffled the wit of man. 



I have placed before you a subject that involves physical and physiological 

 considerations of extreme difficulty. I have endeavoured to show the nature of 

 these diflSculties, and although I have not attempted to solve them all, I have at 

 all events sought to show reasons why we should refer our different colour sensa- 

 tions to differences in the nerve vibrations transmitted from the optic terminals 

 rather than to specifically different activities of cells in the vision centre. Although 

 I have no hesitation in adopting a tetrachromic in place of a trichromic theory of 

 normal vision, I am unable to relinquish the essential part of Young's tlieorj- — that 

 tl'.ere is a relatively small number of primary colour sensations. To abandon that 

 idea entirely, as Krenckel and Berry - have done, is to return to the Newtonian 

 theory, the difficulties of which none knew better than Thomas Young. I have not 

 found it an agreeable task to point out the shortcomhigs of some theories advanced 

 by those for whom I have the deepest regard ; but in the progress of scientific 

 thought it is especially necessary to keep our minds free from the thraldom of 



' Wilson, liesearches on Colour-Miiidness, Edinburgh, 1855, p. 179. 



- G. Berrv. ' Examination of Cases of Colour-blindness,' £dm. Mrd. Jour., Oct. 

 1879, and 'Critical Remarks on the Theories of Fundamental Colour Sensations,' 

 London Ophthalmic Hospital Reports, 1890-91. 



