172 SECTIONAL ADDRESSES 
Sensation of white cannot be produced by any single unitary physical 
stimulation. It requires the simultaneous action of light from more than 
one region of the spectrum. ‘This seems to me a fundamental considera- 
tion, because if a simple sensation like white can be produced only by a 
heterogeneous stimulation, it is possible for a simple sensation like yellow 
to be the result of a heterogeneous stimulation. The sensation of white 
can be produced by stimulation by light from the whole of the spectrum, 
or from three or from two selected regions. There is no fixed standard 
of white. A white surface is one that reflects all visible wave-lengths 
well and equally. In order to define a ‘ white ’ light a standard is taken 
of the radiation of a perfect radiator at 4800° K. or other specified tempera- 
ture.4_ When a white sensation is produced by light from two regions of 
the spectrum, the separate sensations produced by these radiations are 
said to be complementary, and this phenomenon will be referred to later. 
Black sensation cannot be produced by any combination of radiations. 
It is always the result of a relative deficiency of stimulation. A black 
surface is one that does not reflect any visible wave-length to an appre- 
ciable extent. ‘To produce a black effect with spectral lights a brighter 
light must shine, alongside them. ‘Thus, a red produced by wave-length 
of about 6500 A. looks brown when a bright yellow produced by wave- 
length of about 5900 A. shines alongside of it. 
The transition between white and black through grey depends upon 
the relative amount of illumination. ‘There must, however, be the right 
mixture of wave-lengths, otherwise the grey will be tinted with the 
colour sensation produced by those wave-lengths which are in excess. 
We are now in a position to consider the phenomenon of yellow. 
Yellow is a unitary sensation which can be produced by a single group of 
wave-lengths or by two groups, one each on the ‘ red ’ and ‘ green ’ sides 
of the ‘ yellow’ region. If we are to believe that three types of sensory 
mechanism are sufficient to account for colour vision, one of the four 
colours red, yellow, green, and blue must be due to a stimulation of at 
least two of the other ones. For several reasons, yellow has been chosen 
as the heterogeneous one. 
_To my mind there is no more difficulty in considering yellow as due 
to stimulation of two types of receptors than to consider white as due to 
stimulation of more than one type. Experimental evidence supports this 
view. Macdougall,> Rochat,® and others have shown that a ‘red’ 
stimulus to one eye and a ‘ green’ to the other will give a sensation of 
yellow. This result is obtained even with lights from the spectrum. 
The fact has been demonstrated by Hecht,’ but his method is not such a 
satisfactory proof as that obtained by other methods—e.g. a ‘ red’ glass 
over one eye and a ‘ green’ one over the other, or two definite wave- 
lengths of the spectrum each presented to one eye. 
Central summation of this type shows that the sensation is built up in 
the nervous system beyond the optic chiasma, as neither eye need be 
4 W. D. Wright, Proc. Roy. Soc., B, 115, p. 49 (1934). 
5 W. Macdougall, Mind. X.N.S., pp. 52, 210 and 347 (1901). 
° G. F. Rochat, Arch. néerl. de Physiol., 10, p. 448 (1925). 
* S. Hecht, Proc. Nat. Acad. Sci. Wash., 14, p. 237 (1928). 
