460 OCULAR SPECTRA. Sect. XL. ro. 2. 



Hence, when the reverfe fpectrum of any colour became faint, 

 it was wonderfully revived by admitting more light through the 

 eyelids, by removing the hand from before them : and hence, 

 on covering the clofed eyelids, the fpectrum would often ceafe 

 for a time, till the retina became fenfible to the flimulus of the 

 fmaller quantity of light, and then it recurred. Nor was the 

 fpectrum only changed in vivacity, or in degree, by this admif- 

 fion of light through the eyelids ; but it frequently happened, 

 after having viewed 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, 

 except thofe of the object. Thus, when an area of half an inch 

 diameter of pink paper was viewed on a fheet of white paper in 

 the funfhine, the fpectrum with clofed and covered eyes was 

 green •, but on removing the hands from before the clofed eye- 

 lids, 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 palling through the eyelids in great- 

 er quantity than the other colours, induced a yellow fpectrum ; 

 whereas if the fpectrum 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 fpectrum, a greater quantity prevents its formation, 

 as the more powerful flimulus excites even the fatigued parts of 

 the eye into action ; otherwife we fhould fee the fpectrum of 

 the laft viewed object as often as we turn our eyes. Hence 

 the reverfe fpectra are belt feen by gradually approaching the 

 hand near the clofed eyelids to a certain diftance only, which 

 muft be varied with the brightnefs of the day, or the energy of 

 the fpectrum. Add to this, that all dark fpe£r.ra, as black, blue, 

 or green, if light be admitted through the eyelids, after they 

 have been fome time covered, give reddifh fpectra, for the rea- 

 fons given in Sect. III. Exp. 1. 



From thefe circumftances of the extraneous light coinciding 

 with the fpontaneous efforts of the fatigued retina to produce a 

 reverfe fpectrum, as was obferved before, it is not eafy to gain 

 a direct fpectrum, except of objects brighter than the ambient 

 light ; fuch as a candle in the night, the fetting fun, or viewing 

 a bright object through an opaque tube \ and then the reverfe 

 fpectrum is inftantaneoufly produced by the ad million of fome 

 external light ; and is as inftantly converted again to the direct 

 fpectrum by the exclufion of it. Thus, on looking at the fet- 

 ting fun, on clofing the eyes, and covering them, a yellow fpec- 

 trum 



