I.—PHYSIOLOGY 177 
upon the presence of three types of receptors, it is difficult to see how the 
nerve fibres corresponding to the different sensations can be reduced to 
less than three groups, nor can one imagine how fewer than three types 
of receptors can give rise to three groups of nerve impulses. ‘This is a 
general problem as to what extent simplification or complexity can be 
introduced between receptor organs and the interpreting mechanism in 
the brain. It seems to me that the trichromatic hypothesis implies, as 
stated by Young, the presence of three types of receptors linked with 
three groups of nerve fibres in the optic nerves. 
The tentative conclusion is that, in order to explain the phenomena of 
colour vision, it is necessary to have three groups of nerve fibres passing 
to the brain—one group giving rise to sensation A, a second to sensation 
B, and a third to sensation C. We must discover what wave-lengths 
stimulate A, B, and C respectively, what sensations are produced by 
stimulation of one of these alone, and what is the effect of stimulating 
more than one of these, either to the same degree for each or to different 
ratios of response. Stimulation of the receptors may correspond to 
definite wave-length groups, but there may be a certain amount of 
rearrangement in the retinal synapses. It does not seem probable that 
the number of types of receptors or groups of nerve fibres can be reduced 
below three if frequency of the impulses is to be related to intensity of 
stimulation and if only one kind of impulse can pass up each fibre. It is 
like the solution of simultaneous equations: the number of equations 
must be at least equal to the number of unknowns to be found. That 
seems to be the essence of the trichromatic hypothesis as suggested by 
Young. 
Helmholtz introduced the view that the differentiation is due to the 
presence of three photo-active substances which are acted on by the long, 
medium, and short wave-lengths of the visible spectrum respectively. 
The range of radiation which affects these three substances overlaps so 
that, for example, some rays affect all three of these substances. Up 
to the present there is no definite evidence for the presence of three 
photo-active substances, only one photo-active substance, rhodopsin or 
visual purple, has been found. Apart from this fact the view of three 
photochemical substances such as postulated by Helmholtz does not agree 
with the experimental evidence. For instance, in order to explain 
hypochromatism, it is not assumed that one photo-active substance is 
absent but that the range of activity has shifted so that the one substance 
is activated by the range which was formerly active on the two separate 
substances. It does not seem likely that such a chemical transformation 
would occur. 
Hecht has attempted to modify the Helmholtz view by assuming the 
presence of three substances activated by practically the same range of 
radiant energy. The dissimilarities in Hecht’s curves seem to me to be 
too small to explain the differences in colour sensations. Such views as 
those of Hering are untenable so long as we cling to the idea that a single 
nerve fibre can conduct only one type of impulse. Further, the sensation 
of yellow can be produced by the fusion of impulses from the two eyes: 
hence it is not due to the neutralisation of ‘red’ and ‘ green’ in the 
