Notes 251 



by mixture they destroy each other, and produce a whiteness, 

 or greyness, according as they are more or less perfect ; but 

 when kept distinct, they are found to make each other look 

 more brilliant by being brought close together : and all this is 

 agreeable to what is said in section 1 1 , and in the note to 

 section 14. 



22. Sir Isaac Newton observes that he had never been able 

 to produce a perfect white by the mixture of only two primary 



Fig. 28. 



colours, and seems to doubt whether such a white can be com- 

 pounded even of three. He tells us that one part of red lead 

 and five parts of verdigris composed a dun colour, like that 

 of a mouse ; but there is nothing in all this which militates 

 agamst the explanation here given of the cause of the coloured 

 shadows of bodies ; for even supposing that there did not exist 

 in nature any two bodies of such colours as to form perfect 

 whiteness by their mixture, or, to go still further, supposing 

 that no two prismatic colours of the sun could form a com- 

 pound perfectly white, still the facts and reasonings here stated 

 respecting the mixtures of such colours as are called contrasts 

 are so near the truth that they furnish a satisfactory account 

 of the appearances of the colours of the shadows which we 

 have been considering. The terms by which we are accus- 

 tomed to denominate colours have not a very accurate or precise 



