Sir David Brewster on certain Affections of the Retina. 21 
comparing the luminosity of the dise which it reflects with that 
of the fixed circular spot upon the paper. In these experiments I 
cbserved two interesting facts,—that the reflected disc of ight 
exhibited different colours in different parts of it, and that it was 
more luminous than the beam received upon the white ground 
when no part of it was reflected by the moving card. 
Before I had examined these phenomena more minutely, I 
found that M. Benedict Prevost had published an interesting paper 
in the Mémoires de la Société de Physique et d’ Histoire Naturelle de 
Genéve*, and had anticipated me in the first of the above results. 
He agitated, to use his expression, a piece of white card across a 
sunbeam in a dark room, and he observed that the circular spot 
which it reflected, in place of being white, was white only in the 
centre. Around this white spot was a violet spot, growing deeper 
as it receded from the centre. The violet spot was surrounded 
with a zone of deep indigo, very well defined, and resembling the 
colour of the Viola tricolor. Round the indigo zone was one of 
greenish yellow, equally weil defined, and round it a red shade. 
He considers the ight as thus decomposed into seven principal 
colours ! 
“This phenomenon,” he adds, “is produced by a single pass- 
age of the card across the sunbeam, which proves that it 1s inde- 
pendent of the fatigue of the eye. Nor does it depend immedi- 
ately on the agitation or motion of the card, but only, without 
doubt, on some effect of this motion, and particularly on this, 
that the illuminated area strikes the eye only during a very short 
time ; for if the card is sufficiently large, so that the illuminated 
space does not go out of it, and notwithstanding the agitation 
of it the eye continues always to see it, it appears white as if 
it was seen at rest, and there is no longer any appearance of the 
decomposition of the light.” 
Reckoning from the centre of the disc, the colours observed 
by M. Prevost are as follow :— 
White. 
Violet. 
Deep indigo. 
Greenish yellow. 
Red shade. 
In the experiments which I made, I sometimes agitated a 
narrow slit, or a series of parallel slits, in a black card, across 
a circular aperture 1a another black card, or across an aperture 
in the window-shutter of a dark room; or I looked at a white 
surface, or a white disc through the slits of a revolving disc, as 
described in one of the papers already referred to. By these 
* Vol. mii part:2.) p.. 121. 
