I.—PHYSIOLOGY 175 
we must be able to explain how the distinction between red and green can 
disappear, yet without marked decrease in the visibility of any portion of 
the spectrum. The threshold for light is not necessarily altered, and it is 
possible for hypochromats to see clearly through a filter which allows 
only the red end of the spectrum to pass through. In fact a hypochromat 
who cannot see red geraniums amongst the green leaves can distinguish 
the flowers as light objects against a dark background when looking through 
a red glass filter. 
ANOMALOUS 'TRICHROMATISM. 
In 1881 Lord Rayleigh described a condition known as anomalous 
trichromatism, which is characterised by the fact that various people 
require different proportions of ‘red’ and ‘ green’ to match a fixed 
‘yellow ’ but there seems to be no defect in the recognition of colours. 
This condition has been considered by some people to be the basis of a 
division of hypochromats into two groups, and that there are a series of 
cases ranging from normal vision to complete red-green confusion.” 
Up to the present there has been no satisfactory explanation of the 
condition of anomalous trichromatism, and I am now investigating this 
condition. The explanation may be that the radiation corresponding to 
the sodium flame (5896 A.) looks orange to some and greenish to others. 
If the fixed ‘ yellow ’ looks orange, the appearance suggests that the red 
sensation is relatively more stimulated by the ‘ yellow’ light, and more 
‘red’ would be required in the mixture to make the match, whilst if the 
fixed ‘ yellow’ looks greenish, more ‘ green’ would be required. It is 
not yet known whether a similar phenomenon is shown in matching a 
fixed ‘ blue-green’ with a mixture of ‘ blue’ and ‘ green.’ That some 
such explanation is possible is shown by Gothlin,!* who finds that different 
people do not mark out the same region of the spectrum as yellow. The 
maxima of discrimination mentioned previously are also found to be at 
different wave-lengths for various individuals. 
The sensation of yellow seems to be a crucial problem, as it may be 
recognised at different wave-lengths of radiation, and if it is seen over a 
wide range of wave-lengths the subject has defective colour vision. 
MEANS OF STIMULATION BY LIGHT. 
Stimulation of the retina is due to a photochemical action. That is, the 
radiant energy is absorbed and converted into some other form of energy. 
Joly has ascribed the effect to a photoelectric process, meaning that 
electrons are given off as the result of the radiation. It is difficult to see 
in what way this differs from a photochemical action, as electronic changes 
in organic material accompany chemical change. One cannot compare 
12 A. Guttmann, Zeit. f. Psychol. d. Zinnesorgane, Abt. 2, 42, pp. 24 and 250 
(1908) ; ibid., Abt. 2, 48, pp. 146, 199 and 255 (1909). 
13 G, Fr. Gothlin, Journ. Physiol., 57, p. 181 (1923). 
14 QO. Steindler, Sitzungsber. d. Akad. Wiss. Wien, 115, 2a, p. 39 (1906). H. 
Laurens and F. W. Hamilton, Amer. Journ. Physiol., 65, p. 547 (1923). 
