SECT. XL. 10. 3. OCULAR SPECTRA* 461 



trum is feen, which is the direct fpe&rum of the fetting fun ; 

 but on opening the eyes on the iky, the yellow fpe&rum is im- 

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

 trum of the yellow fun, or the direct fpeftrum 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 motions of the opening and clofing eyelids. Hence, 

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

 be ibmetimes coloured, thefe were probably the direct fpeclrum 

 of the blue iky on the parts of the retina fatigued by the white 

 light of the (tar. [Eflays Phyfical and Literary, p. 81. V. 2.] 



When a direct fpeclrum is thrown on colours darker than 

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

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

 But when a direct fpeclrum is thrown on colours brighter than 

 itielf, it becomes inftantly changed into the reverfe fpeftrum, 

 which mixes with thoie 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 fpeclrum mixes with ev- 

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

 itfeif 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 fpeclrum of blue filk, thrown on white 

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

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

 thrown on yellow, became a green. 



In thefe cafes the retina is thrown into activity or fenfation 

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

 tinues the activity or fenfation which forms the fpeclra ; 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 fpeclrum, like the 

 admiilion of external light on a direct fpectrum, as explained 

 above. When they are darker than the direct fpeclrum, they 

 mix it, their weaker ftimuius being inefficient to induce the re- 

 verfe fpeclrum. 



3. Variation oj fyeftr a in refpeft to number ', and figure y and re- 



miffion. 



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

 not always be kept entirely motionlefs \ hence, on infpecling a 



circular 



