SECT. XL, io. OCULAR SPECTRA. 631 



From thefe circumftances of the extraneous light 

 coinciding with the fpontaneous efforts of the fa- 

 tigued retina to produce a reverfe fpeclrum, as was 

 obferved before, it is not eafy to gain a direct fpec- 

 trum, 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 opake tube ; 

 and then the reverfe fpeftrum is inftantaneoufly pro- 

 duced by the admiflion of fome external light ; and 

 is as inftantly converted again to the direct fpec- 

 trum by the exclufion of it. Thus, on looking at 

 the fetting fun, on clofing the eyes, and covering 

 them, a yellow fpedlrum is feen, which is the direct 

 fpectrum of the fetting fun ; but on opening the 

 eyes on the fky, the yellow fpectrum is immediately 

 changed into a blue one, which is the reverfe fpec- 

 trum of the yellow fun, or the direct fpectrum of 

 the blue fky, or a combination of both. And this 

 is again transformed into a yellow one on clofing 

 the eyes, and fo reciprocally, as quick as the mo- 

 tions of the opening and clofing eyelids. Hence, 

 when Mr. Melvill obferved the feint illations of the 

 ftar Sirius to be fometimes coloured, thefe were pro- 

 bably the direct fpectrum of the blue fky on the 

 parts of the retina fatigued by the white light of the 

 ftar. (Eifays Phyfical and Literary, p. 81. V. 2.) 



When a direct fpectrum is thrown on colours 

 darker than itfelf, it mixes with them j as the yel- 

 low fpectrum of the fetting fun, thrown on the 

 green grafs, becomes a greener yellow. But when 

 a direct fpectrum is thrown on colours brighter 

 than itfelf, it becomes inilantly changed into the re- 

 verfe fpectrum, which mixes with thofe brighter 

 colours. So the yellow fpectrum of the fetting fun 

 thrown on the luminous fky becomes blue, and 

 changes with the colour or brightnefs of the clouds 

 on which it appears. But the reverfe fpectrum mixes 

 with every kind of colour on which it is thrown, 



whether 



