ECT. XL. 8. OCULAR SPECTRA. 623 



The alternate exertions of the retina in the pre- 

 ceding fection refembled the ofcitation or pandicu- 

 lation of the mufcles, as they were performed in 

 directions contrary to each other, and were the con- 

 fequence of fatigue rather than of pain. And' in 

 this they differ from the fucceffive diffimilar exerti- 

 ons of the retina, mentioned in thi^ fection, which 

 referable in miniature the more violent agitations of 

 the limbs in con vu Hive difeafes, as epilepfy, chorea 

 S. Viti, and opifthotonos; all which difeafes are per- 

 haps, at firfl, the confequence of pain, and have 

 their periods afterwards eftabliihed by habit. 



VIII. The retina, after having been excited into aclion 

 by ajlimulus fomewhat greater than the laft menti- 

 onedy falls into a fxed fpafawdic affwn, which con- 

 tinuesfor fome days. 



i. AFTER having looked long at the meridian 

 fun, in making fome of the preceding experiments, 

 till the difks faded into a pale blue, 1 frequently ob- 

 ferved a bright blue fpectrum of the fun on other 

 objects all the next and the fucceeding day, which 

 conftamly occurred when I attended to it, and fre- 

 quently when I did not previoufly attend to it. 

 When I clofed and covered my eyes, this appeared 

 of a dull yellow ; and at other times mixed with 

 the colours of other objects on which it was thrown. 

 It may be imagined, that this part of the retina was 

 become infenfible to white light, and thence a blu- 

 ifh fpectrum became vifible on all luminous objects; 

 but as a yellowifh fpectrum was alfo feen in the 

 clofed and covered eye, there can remain no doubt 

 of this being the fpectrum of the fun. A fimilar 

 appearance was obferved by M. ^pinus, which he 

 acknowledges he could give no account of. (Nov. 

 Com. Petrop. V. 10. p. 2. and 6.) 



The 



