Sect. XL. 7. 2. OCULAR SPECTRA, 455 



2. After looking fleadily on about an inch fquare of pink filk, 

 placed on white paper, in a bright funfhine, at the diftance of 

 a foot from my eyes, and clofing and covering my eye-lids, the 

 fpectrum of the filk was at firft a dark green, and the fpectrum 

 of the white paper became of a pink. The fpectra then both 

 difappeared ; and then the internal fpectrum was blue ; and then, 

 after a fecond difappearance, became yellow, and laftly pink, 

 whilft the fpectrum of the field varied into red and green. 



Thefe fucceilions of different coloured fpectra were not exact- 

 ly the fame in the different experiments, though obferved, as 

 near as could be, with the fame quantity of light and other fim- 

 ilar circumftances ; owing, I fuppofe, to trying too many exper- 

 iments at a time ; fo that the eye was not quite free from the 

 fpe£tra of the colours which were previoufly attended to. 



The alternate exertions of the retina in the preceding feclion 

 refembled the ofcitation or pandiculation of the mufcles, as they 

 were performed in directions contrary to each other, and were 

 the confequence of fatigue rather than of pain. And in this they 

 differ from the fucceflive dilTimilar exertions of the retina, men- 

 tioned in this feclion, which refemble in miniature the more 

 violent agitations of the limbs in convulfive difeafes, as epi] fy, 

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

 at firft, the confequence of pain, and have their periods after- 

 wards eftablifhed by habit. 



VIII. The retina, after having been excited into aclion by a Jlimtilus 

 fomewhat greater than the lajl mentioned, falls into a fixed fpaf- 

 modic aclion, which continues for fome days* 



1. After having looked long at the meridian fun, in making 

 fome of the preceding experiments, till the difk faded into a 

 pale blue, I frequently obferved a bright blue fpectrum of the 

 fun on other objects all the next and the fucceeding day, which 

 conftantly occurred when I attended to it, and frequently when 

 I did not previoufly attend to it. Wheti I clofed and covered 

 my eyes, this appeared of a dull yellow ; and at other times mix- 

 ed with the colours of other objects on which it was thrown. 

 It may be imagined, that this part of the retina was b<?come in- 

 fenfible to white light, and thence a bluifh fpectrum became vif- 

 ible on all luminous objects *, but as a yellowifh fpeclrum was alfo 

 feen in the clofed and covered eve, there can remain no doubt 

 of this being the fpectrum of the fun. A fimilar appearance 

 was obferved by M. ^Epinus, which he acknowledges he could 

 give no account of . (Nov. Com. Petrop. V. 10. p. 2. and 6.) 



The locked jaw, and fome cataleptic fpafms, ajre refembled by 



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