ON COLOUR PHOTOMETRY. 
435 
R-pG -|-V =1567 
(R+G)+V =158-5 
(R+V)+G =157-0 
R+(G-f Y)=159-0 
(R+G +Y)= 156 
The mean of these is 157 '44, and by least squares (R+G-pV) = 15770. 
Many other equally concordant measurements were made; we therefore feel satisfied 
that it is true that the impression caused by mixed light is equal to the sum of the 
impressions of its components, also that the method of measurement adopted is trust¬ 
worthy. If this be true when the ordinary light of a candle or lamp is used for com¬ 
parison with different parts of the spectrum, it must be true when using light of any 
other colour. 
§ XIV. Bering’s Theory. 
According to Hering, accepting as correct an abstract by Dr. Pole of the various 
memoirs on the subject (‘Nature,’ vol. 20, pp. 611, 637, vol. 21, p. 14 (1870)), there 
are six fundamental sensations of colour arranged in pairs black and white, blue 
and yellow, green and red. The substance which causes the sensations of black and 
white receives impressions from the whole of the spectrum, being most receptive to the 
yellow and diminishing in receptivity towards both ends of the spectrum. The blue- 
yellow substance receives impressions from two parts, one blue and one yellow, of the 
spectrum, separated by a spot which receives no impression, and where pure green 
is to be seen. The red-green receives impressions from three parts, one green in the 
middle and one red at each end, the three parts being separated by two spots which 
do not affect this substance. Further, Hering supposes that the members of each 
pair of colours cause what might be called opposite chemical actions, which Dr. Pole 
has translated as “ assimilation ” and “ dissimilation.” When the amount of assimila¬ 
tion in a pair is equal to that of dissimilation no light belonging to the pair is present. 
As the white-black substance is affected by all the spectrum, he finds that “ mixed 
light appears colourless when it acts on the blue-yellow or red-green substance with 
equal D (dissimilative) and A (assimilative) power, for then the effects neutralise one 
another, and the action of the black-white substance alone appears. For this reason 
two kinds of light which, when mixed, give white are not complementary but 
antagonistic; they do not produce white by their combination, but merely destroy 
each other, and leave visible the white which was already there.” As our experi¬ 
ments show that the sum of the intensities of different colours equals that of the 
mixed light, the statement in the above paragraph would appear to require modifica¬ 
tion, as it is evident that if one stimulus destroyed the other, the white light resulting 
from a mixture would be feebler than if the colours were truly complementary. 
Further, in colour-blind sight there should be perception of light in those parts where 
it is proved absent. 
3 K 2 
