169 


COLOUR VISION. 
Benham’s top) lead us to expect that there would 
be a marked discrepancy. However, the author’s 
An Introduction to the Study of Colour Vision. | statements are fully borne out by Ives’s results 
By Dr. J. H. Parsons. Pp. viii+ 308. 
bridge: At the University Press, 1915.) 
MasesOd. mete 
(Cam- 
Price 
HE whole subject of colour vision is ad- 
mittedly very difficult, as the knowledge of 
it involves familiarity with the experimental 
results of a physical as well as of a physiological 
laboratory, with the minute anatomy of the retina, 
and with the latest results of psychological re- 
search. Hitherto, all that is known, and the 
various suppositions that have been made about 
the subject, could only be learnt by a prolonged 
search through innumerable scattered papers on 
physics, physiology, medicine, and psychology in 
various languages, chiefly English and German; 
so that it is with most cordial thanks to the author 
that this résumé of the subject in its present state 
is welcomed. 
The book is divided into three parts: part 1. 
(pp. 1-157) devoted to “‘ The Chief Facts of Normal 
Colour Vision”; part ii. (pp. 158-192), ““The 
Chief Facts of Colour Blindness”; and part 11. 
“The Chief Theories of Colour Vision.” The 
arrangement is admirable, the facts are well 
described with very few omissions, and no one 
after reading the first 191 pages will be surprised 
that so far no theory has been suggested that will 
account for all the facts. Perhaps a few of the 
difficulties of the subject may be indicated. 
A given colour may be determined as is well 
known by its hue, its luminosity, and by its 
degree of saturation. It is not always remembered 
that :— 
“Great increase of intensity of light not only 
alters its hue, but also alters its saturation, so that 
eventually it produces only the sensation of white 
light. It would seem, therefore, that luminosity 
is in some recondite sense an inherent ‘ white- 
ness’ in the colour itself, differing in degrees in 
different spectral colours and varying with the 
intensity of these colours. Clearly we are here 
at the outset face to face with a physiological 
fact of immense importance, and much of the 
difficulty of colour vision is connected with this 
fact” (p. 29). 
There is the obvious difficulty in comparing the 
luminosity of two different coloured lights, arising 
from the fact that they give rise to two different 
impressions, and we are surprised to find that, 
according to the note in the preface, there is no 
discrepancy when the luminosity is measured by 
the equality of brightness method or by the flicker 
NO. 2372, VOL. 95| 

with the flicker photometer. 
On p. 3a sentence that refers to the diffraction 
spectrum has missed correction when the proofs 
were read :— 
“Tt suffers, however, from the disadvantage 
that the spectrum is less bright and less extended 
than the prismatic spectrum, and from the still 
greater objection that the interference spectrum 
is never free from scattered light.” 
Surely the extent of the spectrum depends en- 
tirely upon the fineness of the grating, and. its 
brightness may be increased by increasing the 
size of the grating; and if by the term “inter- 
ference spectrum” is meant a diffraction spec- 
trum, any impurity in it is due to the faultiness 
of the grating. 
We are glad to notice Mr. Parsons’s criticism 
of y. Helmholtz’s spectro-photometer and similar 
instruments in which the intensity of light is 
reduced by means of Nicol prisms; much of the 
German work on the subject is inaccurate owing 
to the polarising methods used. 
With regard to Weber’s law, the important 
point mentioned on p. 20 that “it does not hold 
good for very low or very high intensities of 
stimuli”? is to be borne in mind. Fechner’s law 
is true within the limits that Weber’s law holds. 
Schirmer found that it was true between 1 and 
1000 candle-power illuminations (7.e. the func- 
tion is continuous between these limits), but that 
at first his minimum difference was j+,, while 
after eight days’ practice he could recognise a 
change of ;}, or even less (see Ophth. Rev., vol. 
x., p- 179). Possibly the failure of the law at 
the lower limit might be explained by Fechner’s 
later addition of a term sg to represent the 
“intrinsic light of the retina.” If E denote the 
sensation, and S the stimulus, Fechner’s law 
then takes the form 
E=C log(S + s9) +C’ 
where C and C’ are constants. Mr. Parsons 
makes no reference to this suggested emendation. 
The failure of the law at the upper limit may be 
due to changes in the mechanism verging on the 
pathological. The variation of the minimum dis- 
tinguishable difference not only in each individual, 
but also according to his practice (as in the case 
of Schirmer), necessitates great precautions being 
used in the application of Fechner’s law. 
When the eye has been kept in the dark for 
H 
