tji 4 Dr . Darwin’s Experiments on the 
iEpiNus, in the Nov. Com. Petropol. V. io. ; M. Beguelin, 
in the Berlin Mefrioires, V. II. 1771 ; M. d’Arcy, in the 
Hiftoire de l’Acad. des Scienc. 1765 ; M. de la Hire ; and, 
laftly, the celebrated M. de Buffon, in the Memoires de 
l’Acad. des Scien. who has termed them accidental colours, as if 
fubje&ed to no eftabliftied laws, Ac. Par. 1743. M. p. 215. 
I mu ft here apprize the reader, that it is very difficult for 
different people to give the fame names to various ihades of co- 
lours ; whence, in the following pages, fomething muft be 
allowed if, on repeating the experiments, the colours here 
mentioned lhould not accurately correfpond with his own names 
of them. 
I. ACTIVITY OF THE RETINA IN VISION. 
From the fubfequent experiments it appears, that the retina 
is in an a&ive not in a paffive ftate during the exiftence of 
thefe ocular fpeftra ; and it is thence to be concluded, that all 
vifion is owing to the activity of this organ. 
1. Place a piece of red filk, about an inch in diameter, as in 
fig* x . (Tab. IX.) on a ffieet of white paper, in a ftrong light ; 
look fteadily upon it from about the diftance of half a yard for a 
minute ; then doling your eyelids cover them with your 
hands, and a green fpe£trum will be feen in your eyes, refem- 
bling in form the piece of red filk : after fome time, this fpeq- 
trum will difappear and fhortly re-appear; and this alternately 
three or four times, if the experiment is well made, till at 
length it vanilhes entirely. 
2. Place on a Iheet of white paper a circular piece of blue 
filk, about four inches in diameter, in the funlhine ; cover 
the center of this with a circular piece of yellow filk, about 
three 
