totutar Speft fa of Light and Colours, 329 
To evince this fa£t the following fatisfa&ory experiment was 
made. The prifmatic colours were laid on a circular pafteboard 
wheel, about four inches in diameter, in the proportions de- 
ferred in Dr. Priestley’s Hiftory of Light and Colours, 
pi. 1 2. fig. 83. except that the red compartment was intirely 
left out, and the others proportionably extended fo as to com- 
plete the circle. Then, as the orange is a mixture of red and 
yellow, and as the violet is a mixture of red and indigo, it be- 
came neceflary to put yellow on the wheel inftead of orange, and 
indigo inftead of violet, that the experiment might more 
exa&ly quadrate with the theory it was designed to eftablifti or 
confute ; becaufe in gaining a green fpectrum from a red ob- 
ject, the eye is fuppofed to have become infenfible to red light. 
This wheel, by means of an axis, was made to whirl like a 
top ; and on its being put in motion, a green colour was pro- 
duced, correfponding with great exadtnefs to the reverfe fpec- 
trum of red. 
3. In contemplating any one of thefe reverfe fpedtra In the 
qlofed and covered eye, it difappears and re-appears feveral times 
fucceffively, till at length it intirely vanifhes, like the direct 
fpedtra in fe£t. v. ; but with this additional circumftance, that 
when the fpe&rum becomes faint or evanefeent, it is inftantly 
revived by removing the hand from before the eyelids, fo as to 
admit more light : becaufe then not only the fatigued part of 
the retina is inclined fpontaneoufly to fall into motions of a 
contrary direction, but being ftill fenfible to all other rays of 
light, except that with which it was lately fatigued, is by 
thefe rays at the fame time ftimulated into thofe motions 
which form the reverfe fpe&rum. 
From thefe experiments there is reafon to conclude, that the 
fatigued part of the retina throws itfelf into a contrary mode 
Vol. LXXVI. X x of 
