Permanent Colours of Opake Bodies. 247 



The experiments, made with thin plates of 

 glafs, water, and air, from which Sir Ifaac Newton 

 inferred, that the feveral colours of natural bodies 

 arife from the feveral fizes and denfities of the 

 particles, were executed with fo much fkill and 

 precifion, and have been fo often fuccefsfully re- 

 peated, by later inquirers, that they are firmly 

 eflablifhed. 



But, the do6trine of a refle^live power exifting 

 in the Colouring Particles, does not appear to be 

 deducible from thofe experiments, nor is it con- 

 formable to any fubfequent obfervations. 



Sir Ifaac Newton obferved, that thick plates of 

 glafs yield * rings of colours, in the fame manner 

 as the thin plates, from whofe appearances he 

 deduced this explanation of the colours of natural 

 bodies. 



The Duke de Chaulnes has proved, that none 

 of the colours, exhibited by fuch thick plates, 

 are refleded by the anterior furface of the glafs j 

 but that the rays, after having pafTed through 

 t!ie firft furface, to the farther furface, are thence 

 reflefted back. f. 



May it not from hence be inferred, that, fince ■ 

 the adion of thick plates, in exhibiting colours, 

 is analogous to that of thin plates, the colours, 

 even in thefe, are not produced by reflection from 

 the anterior furface, but by a remoter reflexion, 



* Newton. Opt. L. 11. Part. IV. 

 t Acad. Paris. Ann. 1755. p. 136. 



R 4 and 



