SECTION XI 

 THEORIES OF COLOUR VISION 



THE value of a theory to science is as much due to the fresh lines of research which 

 it indicates, as to the explanation which it offers of the already ascertained facts. The 

 theories of vision therefore are of value in spite of the fact that they do not at the 

 present time offer a complete account of the retina and its functions. 



THE DUPLEX THEORY of von Kries states that there are in the retina two entirely 

 separate mechanisms, namely that used for twilight vision which is colour blind, and 

 that used for day vision which responds to colour. The view that the rods with the visual 

 purple supply the former whereas the cones provide the latter, is already familiar 

 because it has been made the basis of the description already given in previous sections. 

 The evidence on which this opinion is based may with advantage be repeated because 

 of its importance. 



Twilight vision is found in those parts of the retina where there are rods ; it is not 

 found therefore at the fovea centralis (because cones alone are to be found there). If a 

 spectrum be examined it is found that the colour with the greatest luminosity is the 

 green, but that red rays are not seen at all. The form of the luminosity curve is identical 

 with the bleaching curve of visual purple, and this pigment is found only where there 

 are rods. The visual acuity of twilight vision is low, and is explained by the fact 

 that many rods as a rule send their impulses through one and the same nerve fibre. 

 In addition to this experimental evidence, there is the statement that animals (e.g. 

 bats and hedgehogs) and birds (e.g. owls) which are nocturnal in habit, have rods in their 

 retinae and not cones. 



Day vision is found most highly developed in the fovea, from which rods are absent. 

 Not only are the cones at the fovea placed very closely together, but it would appear 

 that each cone connects to one nerve fibre only ; in this way the high visual acuity is 

 explained. Further it is stated that animals (e.g. tortoises) and birds (e.g. hens) which 

 are diurnal in habit, have cones only in their retinae. 



It has been suggested recently that the following modifications should be made 

 in the duplicity theory of von Kries : 



1. That the cones do in certain cases function to some extent in night vision, thus 

 redlining one of their primitive rod characteristics, from which on morphological 

 grounds they appear to have been developed. 



2. That the fovea contains some visual purple, being necessary in order that the 

 cones may function in night vision as above, or possibly for the green sensation of day 

 vision. 



3. That the rods play some pajjt in day vision, adding their response to that of the 

 cones. 



These modifications of the duplicity theory concern detail more than they do the 

 basis of the theory, and do not appear to detract at all from the strength of its position. 

 So that so far as the relative roles of the rods and cones are concerned there would 

 not appear to be any room for speculation. Such is not the case however with regard 

 to colour vision, because, of the various hypotheses that have been so far advanced, 

 none have been found to offer a feasible explanation of all the known facts, or to leave 

 u< other possible alternative. A brief account of the rival theories may be given 

 with advantage. 



YOUNG'S HYPOTHESIS states that there are in the retina three differtnt types 



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