A POPULAR ACCOUNT 



THE THICKNESS OF COLOURED PLATES AND PARTICLES OF AIR, 

 WATER, AND GLASS. 



A comparison of^ this Table with the 

 scale in fig. 46 will give a tolerably ac- 

 curate notion of the reflected tints and 

 their composition. But" it also answers 

 the further purpose of measuring the 

 thickness of a medium too minute to be 

 estimated in any other way. Thus the 

 size of the minute parts of natural bodies 

 may be determined from their colours. 

 " If two or more thin plates be laid upon 

 one another, so as to form one plate 

 equalling them all in thickness, the re- 

 sulting colour may be determined. For 

 instance : Hook observed that a faint 

 yellow plate of Muscovy glass laid upon 

 a blue one constituted a very deep purple. 

 The yellow of the first order is a faint 

 one, and the thickness of the plate ex- 

 hibiting it is 4$, to which add 9, the 



thickness exhibiting blue of the second 

 order, and the sum will be 13|, which 

 is the thickness exhibiting the purple of 

 the third order." 



CHAPTER VII. 

 The Theory of Colours, continued. 



(71.) IN explaining the theory of co- 

 lours, it was shown that the colours 

 of natural bodies arose from an apti- 

 tude in them to reflect the rays of 

 some colours rather than those of 

 others. The experiments and investi- 

 gations noticed in the preceding Chap- 

 ter, now enabled Newton to advance a 

 step further in the inquiry into the 

 causes of the colours of natural bodies. 

 He accordingly proceeds to discuss the 



