174 SECTIONAL ADDRESSES 
fewer than those from the end-organs which were not previously stimu- 
lated; hence the more frequent impulses from the previously less 
stimulated receptors will produce the sensation of the colour comple- 
mentary to the inducing one. 
DEFECTIVE COLOUR VISION. 
Abnormal colour vision may be congenital or acquired. It is not my 
intention to discuss certain defects in colour vision due to disease—e.g. 
tobacco amblyopia. 
Defective colour vision is a condition in which the persons affected 
make mistakes in matching colours. Any explanation of the nature of 
colour vision must be able to explain how certain colours are mistaken. 
The usual form of defective colour vision is congenital, and does not alter 
during life. This is what is generally understood when speaking of 
defective colour vision. 'The defect seems to consist in a decrease in the 
ability to distinguish ‘ red’ from ‘ green,’ and the subjects distinguish 
fewer colours than the normal (euchromat) ; hence they may be spoken 
of as hypochromats. It is very difficult to compare the sensations of such 
cases with those of a normal person, but they are frequently described 
as having blue-yellow vision. Another way of expressing the fact is to 
say that in the spectrum they distinguish blue from not blue, whereas the 
normal person subdivides the not blue into red and green. As ‘ yellow’ 
occupies the region between ‘ red’ and ‘ green,’ the defect is most notice- 
able in the ‘ yellow’ region of the spectrum, especially in the milder 
degrees of the defect. 
Part of the evidence for these statements is that analysis of the mistakes 
made by hypochromats are all explained by a failure to distinguish red 
from green! Further evidence is furnished by observations on colour 
discrimination. 
By measuring the difference in wave-length necessary to cause a differ- 
ence in colour, it is found that normal people have two main maxima of 
discrimination where a difference in colour is recognised for a minimal 
change in wave-length. These maxima of discrimination probably 
indicate where there is a most rapid change in the ratio of stimulation of 
two different types of receptor organs. The hypochromat shows only 
one maximum of discrimination, thus suggesting that he has only two 
types of receptor organs. 
In extreme degrees of this defect the whole range of colours can be 
reproduced for these people by fusion of light from two regions of the 
spectrum. 
The normal maxima are in the ‘ yellow’ and ‘ blue-green’ of the 
spectrum, whilst the hypochromat has only one maximum, that in the 
‘blue-green.’ It appears as if the distinction on each side of the yellow 
had been diminished or lost : hence the failure to distinguish ‘ red ’ from 
‘ green,’ and the whole not blue portion of the spectrum appears more or 
less of one colour. The bearing of this on any theory of vision is that 
ll H. E. Roaf, Quart. Journ. Exp. Physiol., 14, p. 151 (1924). A. B. Follows, 
in the press (1934). H. E. Roaf, in the press (1934). 
