214 SECTIONAL ADDRESSES 



Hess ^^ makes the interesting suggestion that many anomalous cases 

 have been wrongly classed as colour weak because they do not accept the 

 normal equation, when all that this proves is colour inequality, which 

 may result from supernormal sensitivity to one colour. This is the 

 objection Hess urges in connection with the use of the anomaloscope, 

 namely that it detects those with red-green inequality, but does not 

 discover those who are supersensitive to either red or green, or whose 

 sensitivity to both red and green is reduced. 



The normal curve evidently permits of fairly wide deviations. ' S ' 

 marked on the curve above is an anomalous trichromate because the 

 excess of green required by him in determining the Rayleigh equation 

 was very marked. Even when starting from the red end, he went right 

 through equality to the green side, requiring considerably more green 

 than normal. Nevertheless, he comes into the normal curve quite easily. 

 It is curious to note that this subject had apparently a very low threshold 

 for green, 10-5 as measured on the scale attached to the Drever-Hilger 

 Hue-Discrimination Spectrometer. The average threshold for 30 subjects 

 for green (5350 A) using the method of serial groups for ten trials, worked 

 out at 17-9 on the scale, the range of the readings^for the subjects being 

 from 5*4 to 30-8. His threshold for red (6700 A), on the other hand, 

 was higher than the average 25 • 6, as compared with 19-2, the range of the 

 same subjects being from 7-0 to 29-0. 



In the article already referred to by Pitt, the question is asked if 

 anomalous trichromates form an intermediate stage between normal 

 trichromates and dichromates. The writer quotes the well-known fact 

 that the equation obtained by the normal does not hold for the anomalous 

 trichromate and indicates that this may be a deciding factor for the 

 negative. But he further argues, in terms of the Young-Helmholtz 

 theory, that ' if the green curve moved over to the red until they coincided, 

 the green curve would in effect have disappeared, and the dichromatic 

 mixture curves would then become a reduced form of trichromatism, 

 although in the intermediate stages the system would be a different rather 

 than a reduced form.' 



It is a moot point as to whether these anomalous trichromates form 

 ' dangerous colour blinds.' The general finding seems to be in favour of 

 the affirmative. Some writers, for example, Troland, actually include 

 them in classifications of types of colour blindness. Oblath ^ points out 

 that they can only recognise colours when they are saturated and of 

 intense luminosity. ' It is evident that these peculiarities render these 

 subjects less fitted for certain services.' In a report on Colour Vision 

 Requirements in the Royal Navy,^^ it is stated that ' the mildly anomalous 

 trichromate can be considered a safe look-out. On the other hand, the 

 unfit anomalous trichromate is, in many ways a greater source of danger 

 than the dichromate.' The incidence of this anomaly is estimated, in 

 the same report, as 6 per cent., which is stated to be a very conservative 



'* Farbenlehre : Ergebnisse der Physiologic, 1922, 20, 1-107. 

 ** Colour Vision Tests. International Labour Office. Studies and Reports, 

 Series F, No. 12, 1929. 



^^ Med. Res. Council Report, Special Series, No. 185, 1933. 



