I.—PHYSIOLOGY 179 
violet ? On the whole, it seems simpler to interpret it as stimulation of 
rods by any wave-length. Furthermore, diseases involving the rods lead 
to night blindness or raising the threshold of achromatic scotopic vision. 
If this threshold is sufficiently raised then there is loss of vision for violet, 
so that the distinction between green and blue is lost.2® This defect is 
a true violet blindness, because it is accompanied by a raised threshold 
for the short wave-length end of the spectrum. Finally, adaptation to 
light conditions is accompanied by a special raising of the threshold to 
the short wave-length end of the spectrum. Therefore, although the 
point is not proved, there is much evidence in favour of violet vision being 
a function of the rods. 
The second type of receptor is one which is concerned with the not 
blue aspect of vision of the hypochromat. ‘These may be cones of which 
there need be only one variety for the hypochromat. 
The third type of receptor would be functional in normal vision, and 
it seems as if this second variety of cone were one that distinguishes red 
from not red, and according to the activity of this variety the stages 
between normal vision and complete red-green confusion can be 
bridged. 
Therefore, normal vision may be due to a receptor which gives rise to 
a red sensation, one which gives rise to a blue sensation and one which 
gives rise to a not blue, not red sensation which, of course, corresponds 
to green sensation. ‘The actual wave-lengths of radiation that stimulate 
the several receptors are not known. The real difference between various 
hypotheses is the extent and region of the spectrum which stimulates the 
end-organs. 
In the Young-Helmholtz hypothesis the type of receptor responsible 
for the sensation of red is stimulated by almost the whole of the spectrum, 
but most strongly by the long wave-length end. ‘The receptors for green 
are stimulated by almost the whole spectrum, but most strongly by the 
mid-region. And those for blue are stimulated by a large extent of 
the spectrum, but most strongly by the short wave-length end. é 
“ Red’ light of longer wave-length than 6200 Avis supposed to stimulate 
the red receptor only, whilst shorter wave-lengths will stimulate the red 
receptor to decreasing degrees, but the other receptors to increasing 
extent, hence the change of colour with wave-length. 
RELATION OF WAVE-LENGTH DIFFERENCES TO COLOUR DISCRIMINATION. 
The change of colour is probably most noticeable when the change in 
ratio of stimulation of the receptors is most marked—e.g. yellow sensation 
might correspond to a sudden decrease in frequency of impulses from 
the receptors for red, a sudden increase in impulses from the receptors 
for green, or a rapid decrease of the former and rapid increase of the latter. 
This assumption is one reason for the great interest in the maxima of 
discrimination in the spectrum. 
20 H. Kollner, Die Stévungen des Farhensinnes (S. Karger, Berlin, 1912). 
