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CHAPTER VII. 



DISCOVERY OF THE LAWS OF THE COLOURS OF 

 THIN PLATES. 



THE facts which we have now to consider are 

 remarkable, inasmuch as the colours are pro- 

 duced merely by the smallness of dimensions of the 

 bodies employed. The light is not analyzed by 

 any peculiar property of the substances, but dis- 

 sected by the minuteness of their parts. On this 

 account, these phenomena give very important indi- 

 cations of the real structure of light ; and at an 

 early period, suggested views which are, in a great 

 measure, just. 



Hooke appears to be the first person who made 

 any progress in discovering the laws of the colours 

 of thin plates. In his Micrographia, printed by 

 the Royal Society in 1664, he describes, in a de- 

 tailed and systematic manner, several phenomena 

 of this kind, which he calls "fantastical colours." 

 He examined them in Muscovy glass or mica, a 

 transparent mineral which is capable of being split 

 into the exceedingly thin films which are requisite 

 for such colours; he noticed them also in the fis- 

 sures of the same substance, in bubbles blown of 

 water, rosin, gum, glass ; in the films on the surface 

 of tempered steel; between two plane pieces of 



