ON COLOUR PHOTOMETRY. 
533 
early to record the method by which the character and amount of the absorption of 
the yellow spot was ascertained. 
A white spot, very feebly illuminated, was placed six inches from the patch on 
which the beams to be compared were thrown. One eye was closed and the other 
directed centrally to the white spot, the observer being at a distance of 4 feet from 
it. The image of the patch was thus received on a part of the retina beyond the 
boundary of the yellow spot. It may appear strange to others, as it did to ourselves 
at first, that the luminosities of the two shadows could be compared with almost 
greater facility than they could be when looked at centrally. When a comparison 
was to be made, the presence of colour often appeared not exactly to vanish but to 
offer no difficulty to the reading. The luminosity was thus determined, and it was 
found practically that the same curve was obtained in whatever angular position the 
white spot was placed, so long as it was six inches from the patch. The luminosities 
of the colours on the patch were also measured when looking directly at them. Any 
difference between the readings by the eye in the two cases showed a lessened or 
increased sensitiveness of the central part of the retina under observation for the 
particular colour. Table I, Col. III., and fig. 33, curve B, gives the results of these 
observations. 
If two square patches of l^-inch side are placed six inches apart, and illuminated 
with white light of the same intensity, and one be looked at centrally, the image of 
the other will fall outside the yellow spot. 
By diminishing the illumination of one or other, the two may be rendered equally 
bright to the parts of the retina used, and by first looking at one centrally then at the 
other, two sets of observations can be obtained. Adopting this plan, and after a large 
number of estimations (it was necessary to take a large number owing to the 
difficulty of the observation), it was found that the relative sensitiveness for white 
light of the centre of the retina to that of the outer part was approximately as 
37 to 33. The areas of the curves of luminosity plotted from the readings are in 
the ratio of 167 to 156, which is so nearly the same ratio that each of their ordinates 
may be taken to indicate the relative amounts of light seen by either part of the 
retina in the different parts of the spectrum. 
Whilst there is, as might be expected, an increase in the luminosity to the outer 
part of the retina of the portion of the spectrum from about E to the violet end, over 
that to the central part of the retina, it is remarkable that the reverse is the case 
with respect to the portion from the green to the red. Evidently, therefore, the 
outer part of the retina is less sensitive than the central part to the less refrangible 
rays of the spectrum. The curve for this part of the retina is very similar to that 
obtained from the observations made with the centre of the eye by persons who 
have a slightly shortened spectrum, and who are, therefore, what is termed partially 
red-blind. 
It should be noted that the luminosity curve given in our former paper, and which 
