6 3 i OCULAR SPECTRA. SECT. XL. ia. 



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, be- 

 comes a deep violet. And the fpectrum of blue 

 iilk, thrown on while paper, was a light yellow ; 

 on black (ilk was an obfcure orange ; and the blue 

 fpectrum, obtained from orange-coLured filk, thrown 

 on yellow, became a green. 



In thefe cafes the retina is thrown into activity or 

 fenfation by the (timulus of external colours, at the 

 fame time that it continues 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 tbefe colours of ex- 

 ternal objects are brighter than the direct fpectrum 

 which is thrown upon them, they change it into 

 the reverfe fpectrum, like the admiflion of external 

 light on a direct fpectrum, as explained above. 

 When they are darker than the direct fpectrum, they 

 mix with it, their weaker ftimulus being inefficient 

 to induce the reverfe fpectrum. 



3. Variation of fpeffra In refpeft to number^ and fgure y 

 and remifllon. 



When we look long and attentively at any ob- 

 je<5t, the eye cannot always be kept entirely motion- 

 lefs ; hence, on infpecting a circular area of red 

 filk placed on white paper, a lucid crefcent or edge 

 is feen to librate on one fide or other of the red 

 circle : for the exterior parts of the retina fome- 

 times falling on the edge of the central filk, and 

 fometimes on the white paper, are lefs fatigued with 

 red light than the central part of the retina, which 

 is conftantly expofed to it ; and therefore, when 

 they fall on the edge of the red filk, they perceive 



it 



