180 SECTIONAL ADDRESSES 
Interpretation of the relations between receptors and incident light is 
not yet attained. Sensation curves merely express the results of matching 
regions of the spectrum with three groups of wave-lengths. As Wright 
remarks,”! ‘ The physiological mechanism by which such an effect could 
be produced cannot be visualised very readily, but it would apparently 
necessitate the assumption that all three fundamental responses have 
some quality in common, so that one response could produce a sub- 
tractive effect on another. ‘This quality must probably be in the nature 
of an inherent ‘‘ whiteness,” and it is on an assumption of this sort that 
saturation differences might be explained.’ 
This view has much in common with the belief of Hering and von 
Kries that there is an underlying white sensation to all stimuli. 
It is possible that monochromatic regions of the spectrum may stimulate 
all three types of receptors to constant ratios—e.g. the extreme ‘ red’ 
end of the spectrum may stimulate all three types to equal degrees or any 
ratio such as 3:2:1. ‘Therefore, the monochromatic ‘ red’ at the end 
of the spectrum may correspond to stimulation of the three types of 
receptors, and not only of one, as represented in Wright’s curve. A 
high degree of discrimination, as in the ‘ yellow,’ would correspond to a 
rapid change in the ratio of stimulation. Therefore, (a) the red sensation 
is rapidly falling off, (6) the green sensation is rapidly increasing, or 
(c) the red is decreasing and green increasing rapidly about 5800 A. 
Similarly a change in the ratio of stimulation is taking place rapidly 
about 4900 Ne 
It is difficult to know how to test these assumptions. The phenomena 
of binocular rivalry, etc., indicate that nerve impulses may be suppressed 
before they produce consciousness: hence sensations may not always 
correspond to the algebraical sum of nerve impulses—e.g. an object seems 
darker when a semi-transparent screen is placed in front of one eye than 
if the one eye is entirely obscured. 
CONDITIONS NECESSARY FOR THE INVESTIGATION OF THE SPECIFIC STIMULI 
FOR VISUAL RECEPTORS. 
For the purpose of finding out what range of wave-lengths is effective on 
the different receptors, weak stimuli must be employed. The eye must be 
in a condition of dark adaptation, because any other state is accompanied 
by stimuli which make the results more difficult to interpret. ‘ White’ 
light should never be used, as it stimulates all receptors; therefore 
specific relations between receptors and stimulus are upset. 
With stronger stimuli a wider range of radiation will become effective 
in stimulating the end-organs ; in fact, with strong illumination it is known 
that the purity of the sensation diminishes, thus showing that weak 
illumination is better for the purpose of differentiating the relation of 
receptors to different wave-lengths of radiation. 
The effect of one group of wave-lengths on the sensitivity of the same 
21 W. D. Wright, Proc. Roy. Soc., B, 115, pp. 69-70 (1934). 
