Dr. Darwin’s Experiments on the 
green fpe&rum ; but they were all at firft red ones : and the 
fame after looking at a candle in the night. 
The reverfe fpedtrum was formed with greater facility when 
the eye was thrown from the object on a fheet of white paper, 
or when light was admitted through the clofed eyelids ; be- 
eaufe not only the fatigued part of the retina was inclined 
fpontaneoufly to fall into motions of a contrary dire&ion ; but 
being ftill fenfible to all other rays of light except that with 
which it was lately fatigued, was by thefe rays flimulated at 
the fame time into thofe motions which form the reverfe fpec- 
trum. Hence, when the reverfe fpedtrum of any colour be- 
came faint, it was wonderfully revived by admitting more light 
through the eyelids, by removing the hand from before them : 
and hence, on covering the clofed eyelids, the fpectrum would 
often ceafe for a time, till the retina became fenfible to the 
flimulus of the fmaller quantity of light, and then it recurred* 
Nor was the fpedlrum only changed in vivacity, or in degree, 
by this admiffion of light through the eyelids ; but it fre- 
quently happened, after having viewed bright objects, that the 
fpeftrum in the clofed and covered eye was changed into a third 
fpe&rum, when light was admitted through the eyelids r 
which third fpedlrum was compofed of fuch colours as could 
pafs through the eyelids, except thofe of the object. Thus, 
when an area of half an inch diameter of pink paper was 
viewed on a fheet of white paper in the funfhine, the fpectrum 
with clofed and covered eyes was green ; but on removing the 
hands from before the clofed eyelids, the fpe&rum became 
yellow, and returned inftantly again to green, as often as the 
hands were applied to cover the eyelids, or removed from 
them : for the retina being now infenfible to red light, the 
yellow rays pafiing through the eyelids in greater quantity 
than the other colours, induced a yellow fpedfrum j whereas if 
