of Edinburgh, Session 1877-78. 
655 
spectrum — the method of reflection, Czermak’s modification of 
Schemer's experiment, the use of rotating disks having coloured 
sectors, and the direct mixture of coloured powders or coloured 
liquids. In all of these cases the effect may be seen with one eye, 
and is due to the action of light on a definite portion of one retina. 
But may sensations of mixed colours be produced by binocular 
vision of the components ] Regarding this question various well- 
known observers have arrived at completely opposite results. Thus, 
as mentioned by Helmholtz in his “ Optique Physiologique,” p. 976, 
H. Meyer, Volkmann, Meissner, Funke, and he himself fail in ob- 
taining the sensation of the resulting colour, whilst Dove, Regnault, 
Briicke, Ludwig, Panum, and Hering state the reverse. In his 
great work, Helmholtz describes various methods by which he 
investigated the question, and his opinion amounts to this, that we 
have no true binocular perception of colour. According to him we 
may have a resultant sensation of a particular kind, different from 
that of the two components, but also unlike the sensation of the 
mixed colour obtained by methods appealing to one eye only. 
In studying this subject I lately devised the following simple 
arrangement : — Take two No. 3 eye-pieces of Hartnack’s microscope, 
or any similar eye-pieces of considerable focal length, and place one 
before each eye. If they be somewhat diverged, two luminous fields 
will be seen, and by adjustment, the edge of the one luminous field 
may be caused to touch the edge of the other. In these circum- 
stances a definite area of each retina is illuminated. By converg- 
ing the eye-pieces, the two fields may then be partially overlapped, 
and when the axes of the two eye-pieces are parallel, both fields 
coincide. It will then be found that the overlapped portion is in- 
tensely luminous, whilst the other portions become less luminous, 
as if cast into shadow. By increasing or diminishing the amount of 
convergence of the eye-pieces, the extent of the luminous field may 
be varied at pleasure, and the two fields coincide when the two 
images fall on the two yellow spots. If, then, a small piece of 
coloured glass be inserted into each eye-piece, say red into one and 
blue into the other, on repeating the experiment as above mentioned, 
I find that the overlapping portion of the two fields gives a sensa- 
tion of the resultant colour. I have repeated the experiment with 
various coloured media, such as coloured gelatine paper, coloured 
