PHYSICAL AND LITERARY. 73 



that the purity of its red colour will not be 

 fenfibly impaired. But, to (hew that, in- 

 proper circumftances, the blueifh colour of 

 fky-light may be feen on bodies illuminated 

 by it, as it is objeded fliould always hap- 

 pen*; expofe to the fun -beams, on a clear 

 cloudlefs day, a iheet of white paper, and 

 place on it any opaque body ; you will per- 

 ceive that the fpace of the {hadow, which is 

 illuminated only by the fky, appears remark- 

 ably blueifh, compared with the reft of the 

 paper which receives the fun's diredt rays. If 

 certain white and black paints mixed together 

 j3roduce blue, it is becaufe the black is not 

 perfed: ihade, but a dark blue or purple f p 

 Any mixture of whitenefs and true black 

 can 'only form a fainter white or grey, which 

 has no more affinity with blue than with red 

 or any other colour, 



QuER. XV. Is not the opinion which Sir 

 Jtfaac Newto?2 feems to have had ||, and, fince 

 him, the generality of philofophers, con- 

 cerning the caufe of the various colours re- 



fleded 



* Mufchen. Phyf. § 1403. 



f Ibid. § 1 172. 



X Neivt. Opt. book 2. part 3. prop. 7. 



il Opt. book z. part 3. prop. 5. near the en^. 



