Sect. XL. 10.3. OCULAR SPECTRA. 461 



trum is feen, which is the direct fpectrum of the fetting fun ; 

 but on opening the eyes on the Iky, the yellow fpectrum is im- 

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

 trum of the yellow fun, or the direct fpectrum of the blue fkv, 

 or a combination of both. And this is again transformed into 

 a yellow one on doling the eyes, and fo reciprocally, as quick 

 as the motions of the opening and doling eyelids. Hence, 

 when Mr. Melvill obferved the fcintillations of the ftar Sirius to 

 be fometimes coloured, thefe were probably the direct fpectrum 

 of the blue (ky on the parts of the retina fatigued by the white 

 light of the ftar. (EfTays Phyfical and Literary, p. 81. V. 2.) 



When a direct fpectrum is thrown on colours darker than 

 itfelf, it mixes with them ; as the yellow fpectrum of the fet- 

 ting fun, thrown on the green grafs, becomes a greener yellow. 

 But when a direct fpectrum is thrown on colours brighter than 

 itfelf, it becomes inftantly changed into the reverfe fpectrum, 

 which mixes with thofe brighter colours. So the yellow fpec- 

 trum 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 ev- 

 ery kind of colour on which it is thrown, whether brighter than 

 itfelf or not : thus the reverfe fpectrum, obtained by viewing a 

 piece of yellow filk, when thrown on white paper, was a lucid 

 blue green ; when thrown on black Turkey leather, becomes a 

 deep violet. And the fpectrum of blue (ilk, thrown on white 

 paper, was a light yellow ; on black filk was an obfeure orange ; 

 and the blue fpectrum, obtained from orange-coloured filk, 

 thrown on yellow, became a green. 



In thefe cafes the retina is thrown into activity or fenfation 

 by the ftimulus of external colours, at the fame time that it con- 

 tinues the activity or fenfation which forms the fpectra ; in the 

 fame manner as the prifmatic colours, painted on a whirling 

 top, are feen to mix together. When thefe colours of external 

 objects are brighter than the direct fpectrum which is thrown 

 upon them, they change it into the reverfe fpectrum, like the 

 admiiTion of external light on a direct fpectrum, as explained 

 above. When they are darker than the direct fpectrum, thev 

 mix it, their weaker ftimulus being infufficient to induce the re- 

 verfe fpectrum. 



3. Variation of fpcElra i?j refpeB to number, and figure y and re- 



miffton. 



When we look long and attentively at any object, the eye can- 

 not always be kept entirely motionleis ; hence, on inflecting a 



circular 



