Mr Smith on the Jluctuatmg Sensibility of the Retina. 95 
naked, I found that the wall appeared brighter to the former 
than to the latter. 
I select the following experiment out of a very great number 
which might be given, to prove that the increased sensibility to 
the greater light, and the diminished sensibility to the lesser, 
above shown, are not to all kinds of light indiscriminately, but 
only to the same primitive colour or colours. 
Experiment S. — I took several slips of thin writing paper, 
four inches long, and three inches broad. One of these I paint- 
ed on both sides red ; another yellow ; a third green ; a fourth 
blue, &c. By applying a black tube to one eye, and one of the 
coloured tubes to the other, I could compare together the diffe- 
rent appearances, if any, which a white surface assumed when 
viewed through these different tubes. I could also compare the 
different appearances of the same surface as seen through any 
two of these coloured tubes at once. ^ In either way the effects 
were very striking. The same white surface, which through a 
black tube appeared bright white, appeared greenish through 
a red tube ; purple through a yellow ; red through a green, &c. 
As my object is to give a faithful account of the principal re- 
sults of my observations upon the state of the sensibility of the 
retina at the time of, and after, viewing surfaces unequally illu- , 
minated, I must not omit the following singular one. 
Experiment 4.^ — In the centre of a thin board I cut a circular 
hole, one inch in diameter. Having covered one side of the 
board with thin writing paper, I painted all of it, except so 
much as extended over the hole, of a deep black colour. The 
circular part which remained white I wetted with olive oil, to 
enable it to transmit light more freely. Thus prepared, I 
placed the board perpendicular to the horizon, between my eyes 
and the flame of a candle, the oiled circle being about an inch 
from, and directly opposite the flame. Then fixing my eyes 
steadily on the centre of the brilliant white, or more correctly 
speaking, yellowish circle, it appeared in the course of some se- 
conds to be surrounded by a broad purple halo, which also was 
surrounded in most instances by a yellowish halo. 
That the part of the retina contiguous to that upon which 
the rays of light from the yellowish circle fell, was, by their ac- 
tion, rendered less sensible to yellow light, the former experi- 
