SECT. XL. 6. OCULAR SPECTRA. 621 



except that the red compartment was entirely left 

 out, and the others proportionably extended fo as 

 to complete 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 became neccflary 10 

 put yellow on the wheel inftead of orange, and 

 indigo inftead of violet, that the experiment might 

 more exactly quadrate with the theory it was de- 

 figned to eftabliih or confute ; becaufe in gaining a 

 green fpcctrum from a red object, the eye is fup- 

 pofed 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 co- 

 lour was produced, correfponding with great exact- 

 nefs to the reverfe fpedtrum of red. 



3. In contemplating any one of thefe reverfe fpec- 

 tra in the clofed and covered eye, it difappears and 

 re-appears feveral times fucceflively, till at length it 

 entirely vanifhes, like the direct fpectra in Sect. V.; 

 but with this additional circumftance, that when the 

 fpectrum becomes feint or evanefcent, it is inftantly 

 Tevivcd by removing the hand from before the eye- 

 lids, fo as to admit more light : becaufe then not 

 only the fatigued part of the retina is inclined fpon- 

 taneoufly to fall into motions of a contrary direc- 

 tion, but being (till fenfible to all other rays of 

 light, except that with which it was lately fatigued, 

 is by thefe rays at the fame time Simulated into 

 thofe motions which form the reverfe fpeftrum. 



From thefe experiments there is rcafon to con- 

 clude, that the fatigued part of the retina throws 

 itfelf into a contrary mode of action, like ofcitation 

 or pandiculation, as foon as the ftimulus which has 

 fatigued it is withdrawn ; and that it ftill remains 

 fenfible, that is, liable to be excited into action by 

 any other colours at the fame time, except the co- 

 lour with which it has been fatigued, 



VII. The 



