
199 

On Colour-Vision by Very Weak Light. 
By Grorce J. Burcu, M.A., D.Sc. Oxon.; F.R.S.; Lecturer in Physics, 
University College, Reading. 
(Received March 7,—Read April 13, 1905.) 
In my paper on Artificial Temporary Colour-Blindness, I wrote :— 
“ Hering’s black-white sensation I have not found, but the evidence for and 
against it is of a somewhat different character, and I propose to discuss it in 
a separate paper.” 
Hering’s theory, as is well known, rests partly on the statement that, 
under a very feeble illumination, all colours appear grey—that there is an 
interval between the chromatic threshold and the absolute threshold of light 
sensation—and his statement is supported by a number of experiments by 
various observers, which have been received as classical. 
I find myself under the necessity of traversing this statement, inasmuch 
as in my own case, to the perfectly rested eye, red, green, blue, and violet are 
instantly recognised as colours and distinguished from each other, and there 
is no interval between the chromatic and the absolute threshold, and it is 
only when the eye is not perfectly rested that all colours appear grey in a 
feeble light. 
In laying before the Society an account of the experiments which have led 
me to this conclusion I have endeavoured, in view of the direct conflict 
between my testimony and that of so many previous writers, to give sufficient 
details to justify my own position, and to indicate a possible explanation of 
these contradictory results. 
Early Hxpervments. 
Experiment 1.—The mental attitude of the observer, of comparatively little 
interest in other cases, has a certain importance when subjective phenomena 
are under investigation. For this reason my first experiment is described in 
greater detail than would otherwise be given. 
I had been repeating, in the course of my studies, and with some success, 
as many as possible of the experiments referred to in my books. Among 
others was the statement that, with sufficiently feeble illumination, all colours 
appear grey. In order to observe this, I arranged a metal chimney, furnished 
with a cap, over a bunsen burner so as to completely prevent the escape of 
