45 4 OCULAR SPECTRA. Sect. XL. 6. 3. 



violet is a mixture of red and indigo, it became neceflary to put 

 yellow on the wheel inftead of orange, and indigo inftead of vi- 

 olet, that the experiment might more exactly quadrate with the 

 theory it was defigned to eftablifh or confute *, becaufe in gain- 

 ing a green fpectrum from a red object, 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 produced, correfponding with great 

 exactnefs to the reverfe fpectrum of red. 



3. In contemplating any one of thefe reverfe fpectra in the 

 clofed and covered eye, it difappears and reappears feveral times 

 fucceffively, till at length it entirely vanifhes, like the direct 

 fpectra in Seel. V. ; but with this additional circumftance, that 

 when the fpectrum becomes faint or evanefcent, 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 fpectrum. 



From thefe experiments there is reafon to conclude, 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 which has fatigued it is withdrawn ; and that it ftill remains 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 col- 

 ours at the fame time, except the colour with which it has been 

 fatigued. 



VII. The retina, after having been excited into ablion by aflimulus 

 fomeiuhat greater than the lafl mentioned \ falls into various fuc- 

 ceffive fpafmodic aclions. 



1. On looking at the meridian fun as long as the eyes can 

 well bear its brightnefs, the diik firft becomes pale, with a lu- 

 minous crefcent, which feems to librate from one edge of it to 

 the other, owing to the unfteadinefs of the eye ; then the whole 

 phafis of the fun becomes blue, furrounded with a white halo ; 

 and on clofing the eyes, and covering them with the hands, a 

 yellow fpectrum is feen, which in a little time changes into a 

 blue one. 



M. de la Hire obferved, after looking at the bright fun, that 

 the imprefiion in his eye firft afTumed a yellow appearance, and 

 then green, and then blue ; and wifhes to afcribe thefe appear- 

 ances to fome affection of the nems. (Porterfield on the Eye, 

 Vol. I. p. 343.) 2. After 



