76 
PEOFESSOE J. CLEEK MAXWELL ON 
colours at different points of the spectrum. The curve marked (R) indicates the inten- 
sity of the red or (24), (G) that of green or (44), and (B) that of blue or (68). The 
curve marked (S) has its ordinates equal to the sum of the ordinates of the other three 
cur\'es. The intensities are found by dividing every colour-equation by the coefficient of 
the colour on the left-hand side. Fig. 6 represents the results of observations by K, 
and fig. 7 represents those of J. It will be observed that the ordinates in fig. 7 are 
smaller between (48) and (56) than in fig. 6. This indicates the feeble intensity of 
certain kinds of light as seen by the eyes of J, which made it impossible to get observa- 
tions of the colour (52) at all without making the slit so wide as to include all between 
(48) and (56). 
This blindness of my eyes to the parts of the spectrum between the fixed lines E and 
F appears to be confined to the region surrounding the axis of vision, as the field of 
view, when adjusted for my eyes looking directly at the colour, is decidedly out of 
adjustment when I view it by indirect vision, turning the axis of my eye towards some 
other point. The prism then appears greener and brighter than the mirror, showing 
that the parts of my eye at a distance from the axis are more sensitive to this blue-green 
light than the parts close to the axis. 
It is to be noticed that this insensibility is not to all light of a green or blue colour, 
but to light of a definite refrangibility. If I had a species of colour-blindness rendering 
me totally or partially insensible to that element of colour which most nearly corresponds 
with the light in question, then the light from the mirror, as well as that from the 
prism, would appear to me deficient in that colour, and I should still consider them 
clrromatically identical ; or if there were any difierence, it would be the same for all 
colours nearly the same in appearance, such as those just beyond the line F, which 
appear to me quite bright. 
We must also observe that the peculiarity is confined to a certain portion of the retina, 
which is known to be of a yellow colour, and which is the seat of several ocular pheno- 
mena observed by Puekinje and Wheatstone, and of the sheaf or brushes seen by 
Haidingee in polarized light; and also that though, of the two observers whose results 
are given here, one is much more affected with this peculiarity than the other, both are 
less sensible to the light between E and F than to that on either side; and other 
observers, whose results are not here given, confirm this. 
§ XV. Exjplanation of the Differences between the two Observers. 
I think, therefore, that the yellow spot at the foramen centrale of Soemmeeing will be 
found to be the cause of this phenomenon, and that it absorbs the rays between E and F, 
and would, if placed in the path of the incident light, produce a corresponding dark band 
in the spectrum formed by a prism. 
The reason why white light does not appear yellow in consequence, is that this absorb- 
ing action is constant, and we reckon as white the mean of all the colours we are accus- 
tomed to see. This may be proved by wearing spectacles of any strong colour for some 
