630 OCULAR SPECTRA. SECT. XL. 19, 



fmaller quantity of light, and then it recurred. Nor 

 was the fpectrum only changed into vivacity, or in 

 degree, by this admiffion of light through the eye- 

 lids ; but it frequently happened, after having view- 

 ed bright objects, that the fpectrum in the clofed 

 and covered eye was changed into a third fpectrum, 

 when light was admitted through the eyelids : which 

 third fpectrum was compofed of fuch colours as 

 could pafs through the eyelids 3 except thofe of the 

 object. Thus, when an area of half an inch dia- 

 meter of pink paper was viewed on a meet of white 

 paper in the funmine, the fpectrum with clofed and 

 covered eyes was green ; b\it on removing the hands 

 from before the clofed eyelids, the fpectrum became 

 yellow, and returned inftantly again to green, as 

 often as the hands were applied to cover the eye- 

 lids, or removed from them : for the retina being 

 now infenfible to red light, the yellow rays pafling 

 through the eyelids in greater quantity than the 

 other colours, induced a yellow fpeclrum ; whereas 

 if the fpe&rum was thrown on white paper, with 

 the eyes open, it became only a lighter green. 



Though a certain quantity of light facilitates the 

 formation of the reverfe fpeclrum, a greater quan- 

 tity prevents its formation, as the more powerful 

 fthnulus excites even the fatigued parts of the eye 

 into acYion ; otherwife we (liould fee the fpeclrum 

 of the laft viewed object as often as we turn our 

 eyes. Hence the reverfe fpedlra are beft feen by gra- 

 dually approaching the hand near the clofed eyelids 

 to a certain diftance only, which mud be varied with 

 the brightnefs of the day, or the energy of the fpec- 

 trum. Add to this, that all dark 'fpedha, as black, 

 blue, or green, if light be admitted through the 

 eyelids, after they have been fome time covered, 

 give reddifh fpe&ra, for the reafons given in Seel. 

 III. Exp. i. 



From 



