I.—PHYSIOLOGY 171 
reduce the sensory data to the lowest possible number, as the simplest 
mechanism is that requiring the fewest possible components. 
All perceptions can be analysed into qualities of sensation—i.e. we can 
distinguish between sensations of light, sensations of sound, etc. ‘These 
qualities can each be subdivided into different attributes of sensation, or 
subqualities. Some of these subqualities are common to all exteroceptive 
systems. 
The subqualities of vision are perception of form or shape, perception 
of movement, recognition of differences in intensity, and discrimination 
of colour. Before we can attempt to distinguish the minimal data which 
must be presented to the cerebrum, an analysis of colour must be made. 
WHAT IS MEANT IN THIS ADDRESS BY COLOUR? 
We can define colour as one of the psychological accompaniments of 
vision. The physicist defines radiation in terms of wave-lengths, but he 
should speak of colour only as a means of avoiding circumlocution. 
When we say ‘ red’ light we use the term ‘ red’ in quotation marks to 
stand for light which gives rise to the sensation of red in the normal 
person. In many cases no difficulty results from the use of such terms, 
but in a description such as this one we must be clear that there is a 
difference between the two uses of the words red, green, etc_—namely, the 
stimulating radiation and the sensation. 
Colours are visible in the spectrum, and we can recognise certain colours, 
which seem unitary and distinct from all others, namely, red, yellow, 
green, and blue. There are, however, other unitary sensations which 
must be considered, namely, white and black : these cannot be produced 
by stimulation with any one region of the spectrum. ‘These two sensa- 
tions are sometimes described as belonging to the colourless sensations, 
but psychologically one cannot separate them from a discussion on 
colour. 
Thus we find that certain colours are related to definite regions of the 
spectrum, but there are other sensations which do not correspond to any 
_ single group of wave-lengths : the latter are the purples, white, and black. 
All colours can be represented by fusion of lights from several regions 
of the spectrum, and the minimum number of regions is three. This 
physical relation is generally considered of paramount importance in the 
discussion of colour vision. 
In 1802 Young postulated that there were three sensory mechanisms, 
because all colours could be reproduced by a combination of three regions 
of the spectrum. ‘There has always seemed some difficulty in recon- 
ciling this view with the psychological standpoint that there are six distinct 
kinds of visual colour sensation, namely, red, yellow, green, blue, white, 
and black. In the discussion of this problem some of these simple 
psychological effects can be shown to be built up from other sensory 
processes. The discussion of the sensation of yellow occupies an 
important place, but before we deal with the sensation of yellow it is 
simpler to consider the sensations of white and of black. 
3 T. Young, Phil. Trans., 92, p. 12 (1802). 
