328 *Dr. Darwin’s Experiments on the 
By the Following obfervations it appears, that a fimilar cir- 
cumftance obtains in the organ of vifion ; after it has been 
fatigued by one kind of action, it fpontaneoufly falls into the 
oppofite kind. 
1. Place a piece of coloured filk, about an inch in diameter, 
on a fheet of white paper, about half a yard from your eyes ; 
look fteadily upon it for a minute ; then remove your eyes 
upon another part of the white paper, and a fpedlrum will be 
feen of the form of the filk thus infpefted, but of a colour 
oppofite to it. A fpe&rum nearly fimilar will appear if the 
eyes are clofed, and the eyelids {haded by approaching the 
hand near them, fo as to permit fome but to prevent too 
much light falling on them. 
Red filk produced a green fpe£trum. 
Green produced a red one. 
Orange produced blue. 
Blue produced orange. 
Yellow produced violet. 
Violet produced yellow. 
That in thefe experiments the colours of the fpe&ra are the 
reverfe of the colours which occafioned them, may be feen 
by examining the third figure in Sir Isa ac Newton’s Optics, 
L. II. p. 1. where thofe thin laminae of air, which reflected 
yellow, tranfmitted violet ; thofe which reflected red, tranf- 
mitted a blue-green ; and fo of the reft, agreeing with the 
experiments above related. 
2. Thefe reverfe fpe£lra are fimilar to a colour, formed by a 
combination of all the primary colours except that with which 
the eye has been fatigued in making the experiment: thus the 
reverfe fpeflrum of red mu ft be fuch a green as would be pro- 
duced by a combination of all the other priimatic colours. 
To 
