22 Sir David Brewster on certain Affections of the Retina. 
methods I found that the white disc, or aperture, was whitest in 
the centre at certain velocities, and when the light was strong, 
but that it was bluish when the light was moderate. The fol- 
lowing was the order and character of the tints, reckoning from 
the centre :— 
White or bluish. 
Darker blue. 
White. 
A dark ring, pretty well defined. 
White. 
Greenish yellow. 
Reddish. 
These colours vary, within certain limits, with the sensibility of 
the eye and the intensity of the light. I have sometimes observed 
the centre of the dise darkish blue, and sometimes yel/ow. When 
the velocity is great and the disc seen distinctly, there is not the 
slightest trace of colour. 
Phenomena somewhat analogous to these may be seen in the 
flame of a spirit-lamp, and in other flames, where the effect is 
caused by the shooting up of the flame, which produces success- 
ive impulses on the retina. At the top of the flame are seen 
several curves, convex upwards, like elongated parabolas. They 
are generally of a sap or olive-green colour ; and sometimes the 
most brilliant red tints appear where the uppermost curve opens 
at its vertex, and leaves two lateral and almost parallel branches. 
Colours similar to those discovered by M. Prevost have been 
recently observed by Mr. John Smith of the Perth Academy*, 
who draws from them what he justly calls many “ startling con- 
clusions.” M. Prevost and this writer have greatly misappre- 
hended the nature of these phenomena. While the Swiss phi- 
losopher considers them as “independent of the fatigue of the 
eye,” and as exhibiting a new decomposition of light by motion, 
Mr. Smith pronounces them to be “ produced by alternate light 
and shade in various proportions ;”” and he regards them “as 
proving the non-homogeneity of zether—as proving the undulatory 
hypothesis, but opposing the undulatory theory—as contrary to 
the idea of the waves of light having different lengths—as help- 
ing to explain many of the phenomena of polarization—and as 
giving a new explanation of prismatic refraction” different from 
that of Newton! 
Although we cannot adopt these conclusions, yet the phe- 
nomena, when carefully studied, as being the effect of suc- 
cessive impulses on the retina, acting in the manner which [ 
* “On the Production of Colour and the Theory of Light,’”’ Report of 
the British Association at Aberdeen, 1859, Trans. p. 22. 
