454 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 eltablifh or confute ; becaufe in gam- 

 ing a green fpettrum from a red objeft, 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 

 jmotion, a green colour was produced, correfponding with great 

 cxatnefs to the reverfe fpeftrum of red. 



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

 clofed and covered eye, it disappears and reappears feveral times 

 fucceflively, till at length it entirely vanilhes, like the direft 

 fpeclra in Se&. V. ; but with this additional circumftance, that 

 when the fpeftrum 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 direftion, 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 intothofe motions which form 

 the reverfe fpedrum. 



From thefe experiments there is reafon to conclude, that the 

 fatigued part of the retina throws itfelf into a contrary mode of 

 aHon, like ofcitation or pandiculation, as foon as the ftimulus 

 which has fatigued it is withdrawn ; and that it flill remains 

 fenfible, that is, liable to be excited into aflion 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 aElkn by ajlimulus 

 fomenvhat greater than the loft mentioned \ falls into various fuc- 

 cejfive fpafmodic aflions. 



? . ON looking at the meridian fun as long as the eyes can 

 well bear its brightnefs, the difk 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 clofmg the eyes, and covering them with the hands, a 

 yellow fpedlrum 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 impreflion in his eye firft aiTumed a yellow appearance, and 

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

 ances to fome aiTeQion of the nerves. (Porterfield on the Eye, 

 Vol. L p. 343.) 2. After 



