LAWS OF THE COLORS OF THIN PLATES. 77 



property of the substances, but dissected by the minuteness of theii 

 parts. On this account, these phenomena give very important indica- 

 tions of the real structure of light ; and at an early period, suggestcG 

 views which are, in a great measure, just. 



Hooke appears to be the first person who made any progress in dis- 

 covering the laws of the colors of thin plates. In his Micrograpkia, 

 printed by the Royal Society in 1664, he describes, in a detailed and 

 systematic manner, several phenomena of this kind, which he calls 

 " fantastical ^colors." He examined them in Muscovy glass or mica, 

 a transparent mineral which is capable of being split into the exceed- 

 ingly thin films which are requisite for such colors ; he noticed them 

 also in the fissures of the same substance, in bubbles blown of water, 

 rosin, gum, glass ; in the films on the surface of tempered steel ; be- 

 tween two plane pieces of glass ; and in other cases. He perceived 

 also, 1 that the production of each color required a plate of determi- 

 ii.ite thickness, and he employed this circumstance as one of the 

 grounds of his theory of light. 



Newton took up the subject where Hooke had left it ; and followed it 

 out with his accustomed skill and clearness, in his Discourse on Light 

 and Colors, communicated to the Royal Society in 1675. He deter 

 mined, what Hooke had not ascertained, the thickness of the film 

 which was requisite for the production of each color ; and in this 

 way explained, in a complete and admirable manner, the colored 

 rings which occur when two lenses are pressed together, and the scale 

 of color which the rings follow ; a step of the more consequence, as 

 the same scale occurs in many other optical phenomena. 



It is not our business here to state the hypothesis with regard to the 

 properties of light which Newton founded on these facts ; the " fits o 

 easy transmission and reflection." We shall see hereafter that hi? 

 attempted induction was imperfect; and his endeavor to account, by 

 means of the laws of thin plates, for the colors of natural bodies, is alto- 

 gether unsatisfactory. But notwithstanding these failures in the specu- 

 lations on this subject, he did make in it some very important steps ; 

 for he clearly ascertained that when the thickness of the plate was 

 about 1-1 78000th of an inch, or three times, five times, seven tunes 

 that magnitude, there was a bright color produced; but blackness, 

 when the thickness was exactly intermediate between those magni- 

 tudes. He found, also, that the thicknesses which gave red and vie- 



1 Microyraphia, p. 53. 



