124 Proceedings of Royctl Society of Ediribiirgli. [sess. 
would not account for tlie colours it showed. He pointed out in 
particular that two of the fundamental colour-sensations would not 
make white, but that their equilibrium must be deeply saturated 
hues of green, blue-green, green-blue, or blue, which is in contradic- 
tion with fact. 
He preferred a more probable assumption. He chose, on reason- 
able grounds, as Maxwell did, blue for one of the three fundamental 
colour -sensations, and considered that the other two, the red and 
the green, might coincide and form yellow. He further showed that 
by a further coincidence of the blue sensation the outer zone, of 
light and shade only, might be explained ; and he gave a diagram 
to show how this might occur. 
In 1879 he published a more elaborate essay on vision generally, 
in which this explanation was further insisted on. 
AB. 
KrencheVs Remarks on Theory^ 1880. 
The author says : — “ It is perhaps possible to bring the several 
physiological and pathological facts in unison with the three funda- 
mental colours of Young and Helmholtz, — even the apparently 
opposing facts seem lately to have been explained, according to this 
elastic theory, by a suitable alteration of the ‘excitement curves.’ 
But it is very suspicious that the facts are explained with equal ease 
by Hering’s theory of two pairs of fundamental colours. And even 
this similar suitability of two different hypotheses seems to point 
to the conclusion that their correspondence with fact does not say 
much in proof of their value. 
A hypothesis is only allowed when it is necessary, i.e., when 
some difficulty in regard to the comprehension of the phenomena 
is removed by it. If the phenomena explain themselves without 
the hypothesis, the latter seems to me unnecessary and unallowable. 
We only know two phenomena of the whole process of colour 
perception — the first cause, 7.e., the coloured rays, and the final 
effect of them, i.e., the colour impressions. All the physiological 
process which lies between them is unknown. How there is nothing 
about fundamental colours in either of these phenomena. The 
spectral colours are all equally unmixed ; the several colour im- 
