1912.] Trichromic Vision and Anomalous Trichromatism. 169 



is frequently associated with colour weakness. The colour weak are also 

 particularly liable to fail in making an equation, but in addition to making 

 the anomalous equation they are in most cases satisfied with that of the 

 normal. Anomalous trichromatism cannot be due to the diminution of a 

 green sensation in the sense of the Young-Helmholtz theory. Apart from 

 the fact that I have shown that yellow is a simple and not a compound 

 sensation (L8, 24, 25, 26), there would be no reason why more green should 

 be required in making the compound yellow, since the simple yellow would 

 also contain less of the hypothetical green sensation. If whilst the yellow 

 remains as in the normal the sensitiveness to green light were diminished 

 •or to red increased we should have an explanation of the facts. Schuster (19) 

 found that the position selected as pure yellow was the same with the green- 

 anomaly as with the normal-sighted. Whilst there are red-anomalies who 

 show weakness for red, there are others who do not, and this may be 

 explained by an increased sensitiveness to green whilst the red and yellow 

 remain as in the normal. 



Summakv. 



1. Trichromic vision is not synonymous with anomalous trichromatism. 



2. Many persons with otherwise normal colour-perception make an 

 anomalous equation. 



3. Many colour-blind persons (dichromics and trichromics) make an 

 absolutely normal match with no greater mean deviation than the normal. 



4. Colour weakness is not characteristic of anomalous trichromatism but of 

 trichromic vision. 



5. Anomalous 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. 



BIBLIOGRAPHY. 



1. Edridge-Green. " Colour Blindness and Colour Perception," ' Int. Scient. Series,' 



1S91 and 1909. 



2. „ "A Trichromic Case of Colour-blindness," 'Ophth. Soc. Trans.,' 



1901. 



3. „ "The Evolution of the Colour Sense," ' Ophth. Soc. Trans.,' 1901. 



4. „ " Two Cases of Trichromic Vision," ' Roy. Soc. Proc.,' 1905. 



