159 
of Edinburgh, Session 1869-70. 
The second observation that may be made upon the results is, 
that by oblique vision the various colours are seen under the same 
conditions as in the most common form of colour-blindness, wherein 
there are really only two colour-sensations, the upper half of the 
spectrum, from blue-green up to violet, and including pinks and 
purples, appearing blue ; and the lower half, from yellow-green 
down through yellow, orange, and scarlet, to bright red, appearing 
yellow ; and in such colour-blindness the extreme red is frequently 
very dull. We may, therefore, expect the discovery of some simi- 
larity in the conditions of the central part of the retina of an eye 
affected with this form of colour-blindness, and the marginal parts 
of the retina of a normal eye. 
Before concluding, I would venture to connect the discovery 
with an existing theory of colour-sensation, as it may help to 
establish that theory, should a prediction the connection leads to 
be found to be correct. 
The figure here given shows a section of part of the retina 
(Kolliker). Now, it has been suggested that 
each of the layers Y, Gf, and B, is receptive of the 
sensation of light, — the layer Y being affected 
by the more refrangible rays blue and violet, B 
being affected by the less refrangible yellow, 
orange, and red, while the central layer Gf is 
affected by the central parts of the spectrum, 
blue, green, yellow, and orange ; and this would 
account for the approximate achromaticity of 
the eye, for when the eye is arranged for the most acute vision, 
the focus of blue rays will correspond with Y, of green rays with 
Gf, and of scarlet rays with B. 
But it is well known that the eye does not see any colour quite 
purely; there is always white light present, or, in other words, one 
of the layers, Y, Gf, or B, cannot be agitated or excited without the 
others partaking to some extent in the excitation. Now, there is 
a probability that the degree of freedom with which one layer may 
transmit its special sensation without one or both of the others 
participating, to an important degree, in the excitement, depends 
in part upon the maintenance of a considerable interval between 
the layers. Let us then imagine the interval between Gf and B to 
