I.—PHYSIOLOGY 181 
area of the retina to another group, is probably the only method of com- 
paring the stimulating actions of these groups on the same receptors. 
If different receptors are acted upon, one light should not affect the 
sensitivity to another, but if the same receptors are concerned, 
then interference will take place according to the Weber-Fechner 
relationship. 
Experiments of the above nature suggest that long wave-lengths of 
visible radiation stimulate all receptors to an appreciable degree, whilst 
the shorter ones act mainly on one only. 
As a result of my own experiments I am led to believe that the ranges of 
wave-length which stimulate the various receptors correspond to the 
effects to be expected from the coloured globules found in the birds’ 
retine. No such colour filters have been found in the eyes of mammals 
higher than the group of marsupials. It may be that photo-active 
substances are the means of selection. 
The three types of receptors would be : (1) Those corresponding to the 
red globules which would be stimulated by the long wave-length end of 
the spectrum, with a marked falling off in effect about 5800 A. As no 
filter is absolutely opaque, it is probable, especially with bright lights, 
that some stimulation of these occurs by wave-lengths to the extreme 
short wave-length end of the spectrum. These receptors would be 
absent or the pigment in the filter reduced in the various degrees of 
hypochromatism. (2) Those corresponding to the yellow globules which 
would be stimulated by long and intermediate wave-lengths, with a 
marked falling off in effect about 4900 A. Some stimulation might also 
be produced by shorter wave-lengths. (3) Those corresponding to the 
pale greenish globules which could be stimulated by the whole of the 
visible spectrum. ‘ Red’ light would thus stimulate all three receptors. 
‘Green’ light would stimulate mainly two. ‘ Violet’ light would 
stimulate mainly one. 
SUMMARY. 
Colour vision is probably dependent upon three types of receptor 
organs. In some persons the activity of one of these types is reduced or 
absent, giving rise to varying degrees of defective colour vision. 
Discrimination curves suggest that the change in ratio of stimulation 
occurs rapidly at wave-lengths about 5800 A. and 4900 A. Hypochromats 
do not possess the maximum near 5800 A.; hence their dichromatic 
vision depends upon two types of receptors with marked change in ratio 
of stimulation by wave-lengths about 4900 A. 
The normal person differs, therefore, from the hypochromat in that 
the former is better able to distinguish wave-lengths of radiation longer 
than 5800 A. from shorter ones. The defect does not appear to be a mere 
absence of one type of receptor leaving a portion of the spectrum unrepre- 
sented, but it seems as if the red discrimination of the euchromat were 
superimposed on a background of something else. In the absence of 
discrimination of ‘ red’ the background might be classed as yellow, but 
