296 COLOUR VISION 



make an anomalous equation. (3) Many colour-blind persons (dichro- 

 mics and trichromics) make an absolutely normal match with no greater 

 mean deviation than the normal. (4) Colour weakness is not charac- 

 teristic of anomalous trichromatism but of trichromic vision. (5) Ano- 

 malous trichromatism and colour weakness are not synonymous. 

 (6) A large mean deviation indicates colour weakness. (7) Anomalous 

 trichromatism appears to be due to an alteration in the normal relations 

 of the response to the three colours (lights) used in the equation. If the 

 eye be more or less sensitive to one of the components of the mixed 

 colour whilst the other has its normal effect, an anomalous equation will 

 result. An anomalous equation will also result when the yellow is more 

 allied to green or red than is normal." 



It is difficult to understand how any concordant classification of 

 the colour-blind can be arrived at by Edridge-Green's methods. His 

 classification is based upon the number of different colours distinguished 

 in the spectrum, yet these may be subdivided into a variable number of 

 monochromatic patches. He says : " They (the physicists) appear to 

 take for granted that the perceptions of others are similar to those 

 experienced by themselves 1 ." Yet there can be no doubt that in 

 practice he is much influenced by the names which the examinee applies 

 to the various parts of the spectrum. " When it (the colour perception 

 spectrometer) is used to test colour blindness, the examinee should first 

 be shown some portion of the interior of the spectrum and then asked 

 to name the various colours which he sees 2 ." Indeed, he constantly 

 lays stress upon the importance of colour names in testing for colour 

 blindness. As has already been pointed out, the colour-blind subject 

 uses a terminology for colours which is suited to describe the perceptions 

 of normal-sighted people, and w T hich is unsuited to describe his own, 

 yet these colour names are apparently regarded by Edridge-Green as 

 an accurate and reliable criterion of what the colour-blind person sees 3 . 



So far as the " retinal " part of Edridge-Green's theory is concerned 

 I think that the evidence is strongly against it. It appears to me to 

 be very improbable that the rods have no direct visual function. 

 Anatomically and embryologically they must be regarded as the primitive 

 visual neuroepithelium, the cones being a more highly differentiated 



1 Colour -Blindness, p. 14, 1909 2 Hunterian Lectures, p. 75, 1911. 



3 To avoid any misconception on this important point I wish to emphasise the fact 

 that these remarks apply to the determination of the iype. of colour blindness of the 

 examinee. In practical testing the object aimed at is the determination of whether the 

 examinee recognises red, green, and white lights as red, green, and white lights respectively, 

 and it is obvious that the names which he applies are of great importance. 



