[August, 
194 Analyses of Books. 
He next points out that whilst according to the current theory 
of colours and colour sensations— that of Young and Helmholtz— 
we have three colour sensations, yet the latter authority admits 
that for the present the theory lacks anatomical basis, as no such 
distinaion of fibres can be traced in the retina. He then pro- 
poses to take two colours of the spearum and to add their wave- 
lengths together, thus obtaining the intermediate length. _ 1 hus 
the & mean between yellow and red is almost identical with the 
wave-length of orange, the intermediate colour. Now it is com- 
monly said that if all the colours of the spedtrum are added 
together they make white light. But if we add together the wave- 
lengths of all the seven colours and take their mean, we get 
0-0000213, which falls between the yellow and the green. T ms, 
the author says, is “just where the eye would lead us to expect 
it. White would therefore appear to be a colour ot the spedtrum, 
occupying the medium place, though not always showing plainly, 
because of the narrowness of its band.” The fadt that the union 
of the blue and yellow rays of the spedtrum produces white and 
not green is pronounced by Helmholtz the most striking diflen 
ence between a mixture of pigments and a mixture of colouied 
rays. Starting from these fadts, Mr. St. Clair disputes the theory 
of three primary colours. He writes: — “Violet and red should 
not be regarded as simple colours on the mere ground that they 
cannot be found by compounding two others. Being at the ex- 
tremities of the scale neither of them is a mean between two 
others — no two wave-lengths in the scale, on being added 
together and divided by two, will yield a value so high as 
0-0000256 (red), or so low as 0-0000174 (violet).” Concerning 
green he quotes Prof. Tyndall, who states that C. E. Wiinsch, on 
mixing yellow with bright blue, obtained a white with a greenish 
tin cr e. 
The ordinary theory of after-images is not free from difficulties. 
“ If we have been looking at a red patch, and then on turning 
our eyes away see a patch of blue-green, we are asked to believe 
that we have fatigued one of the primary colour sensations and 
made ourselves temporarily blind to red, so that in looking at a 
white background the aftedted portion of the retina can only dis- 
cern the green complementary.” Mr. St, Clair, however, brings 
forward an objection which Mr. S. Hodges also entertains “ If 
after we have gazed for thirty seconds at a red patch the retina, 
or any set of rods there, is so exhausted that it can no longer 
perceive red rays, it ought to cease to see the red patch when we 
continue looking at it for more than thirty seconds, which is not 
the case.” Here, however, these writers are both in error. In 
“ matching off ” scarlet, magenta, &c., cloths, dyers find it neces- 
sary to use great dispatch. If the eyes are fixed on such colours 
for any length of time fatigue is certainly expeiienced, and the 
dyer becomes unable to tell whether he has hit the exadt shade 
required or not. 
