LAWS OF OTHER PHENOMENA. 419 



peated or varied these experiments in different 

 ways. 



Newton had noticed certain rings of colour 

 produced by a glass speculum, which he called 

 " colours of thick plates," and which he attempted 

 to connect with the colours of thin plates. His 

 reasoning is by no means satisfactory; but it was of 

 use, by pointing out this as a case in which his 

 "fits" (the small periods, or cycles in the rays of 

 light, of which we have spoken,) continued to occur 

 for a considerable length of the ray. But other 

 persons, attempting to repeat his experiments, con- 

 founded with them extraneous phenomena of other 

 kinds ; as the Due de Chaulnes, who spread muslin 

 before his mirror 6 , and Dr. Herschel, who scattered 

 hair-powder before his 7 . The colours produced by 

 the muslin were those belonging to shadows of 

 gratings, afterwards examined more successfully by 

 Fraunhofer, when in possession of the theory. We 

 may mention here also the colours which appear on 

 finely-striated surfaces, and on mother-of-pearl, fea- 

 thers, and similar substances. These had been 

 examined by various persons, (as Boyle, Mazeas, 

 Brougham,) but could still, at this period, be only 

 looked upon as insulated and lawless facts. 



6 Ac. Par. 1755. 7 Phil. Trans. 1807. 



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