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primogenial colours. Place a piece of coloured filk, about an inch 

 in diameter, on a flieet of white paper, about half a yard from 

 your eyes; look fteadily upon it for a minute; then remove 

 your eyes upon another part of the white paper, and a fpedrum 

 will be feen of the form of the filk thiis ilifpedled, but of a diffe-- 

 rent colour, thus 



Red filk produced a green fpedrum. 



The reafon of thefe phasnomena is very ingenioufly afligned by 

 Dr. Darwin ; he fays, that the retina being excited into a violent 

 and long continued adion by the red rays, in the firft experiment,, 

 at length is fo fatigued as to become infenfible to them ; but that 

 it ftill remains fenfible, that is, liable to be excited into adion by 

 any other colours at the fame time ; and therefore the fpedrum 

 afi\imes a green appearance, becaufe if all the red rays be taken 

 out of the folar light, the remaining rays will compofe green. See 

 Phil. Tranf. Vol. LXXVI. Converfely, a green objed produces a 

 red ocular fpedrum. Now we may obferve, that if all the green 

 rays be taken out of the folar fpedrum of feven colours, the re- 

 maining colours will not compound red. If indeed green be hot 

 a primitive colour,, but a compofition of blue and yellow, then 



will: 



