ON COLOUR PHOTOMETRY. 
445 
appear at first sight that he was blind to red and green, which would correspond to a 
tetrachromatic theory. On the trichromatic theory also he is not only defective in 
perception of red, but also of green. Whether this last view is in accordance with 
his perceptions of colour as he describes them it is difficult to say : but be that as it 
may, we leave the deficiencies noted for theorists to deal with as they like, merely 
suggesting that in dealing with Dr. Pole’s curve the others should also be taken into 
account, more particularly G’s, which indicates a deficiency for red intermediate 
between that of Dr. Pole and those of H and Id. 
§ XVIII. Actual Intensity of Illumination seen by Colour-Blind People. 
The question as regards the intensity of illumination actually seen by colour-blind 
people is one which is very difficult to solve. There appear to us to be two plans by 
which this might be done, but both are open to doubt. The first is by the method of 
extinction by means, say, of a graduated wedge. The difficulties in this plan are 
evident. Our own experience is that the state of health at the time has a marked 
influence on the precision with which the observations can be made, as has also the 
time which the observer has spent in complete darkness before the observation. The 
other method, to which we have already alluded, is that of observing closely ruled 
lines, and estimating the intensity of the light by the distance at which it had to be 
placed from the lines in order that they might just not be “ resolved.” 
This latter plan one of us (Captain Abney) tried with the observer G. Fine 
parallel lines, fairly closely ruled on white paper, were placed about 6 feet from an 
observing telescope, and the two observers, having focussed the lines sharply when 
they were illuminated by a bright light, alternately observed the disappearance of the 
lines as a candle was moved from and toward the screen on which the lines were 
ruled. The following results are the means of several (never less than four) observa¬ 
tions in each case, none of which differed more than one inch from the mean. 
Indies. 
With the lines vertical. 
* -i A 
distance of candle 
108 
lG 
33 
33 
7 2 
horizontal. 
J A 
33 
33 
04 
‘ lG 
33 
3 3 
71 
33 33 
at 45° on 
each f A 
33 
33 
81 
side of the 
vertical 
lG 
33 
33 
72 
A had previously found his eyes to be astigmatic, and these observations would 
have proved it if proof were necessary. It appears from the above that except when the 
lines were horizontal less light was necessary to enable him to resolve them than for G. 
Apparently the lens of G's eye is spherical, and it is probable that although for one 
direction of the lines his definition may be inferior to As, it may be superior for 
