Q<2 METHOD or EXAMINING DISPERSIVE POWERS. 



fiderable utility, on account of the fmallnefs of the quantity 

 requisite for trial. In oil of cloves, for inftance, I have met 

 with a wide difference. The refractive power of genuine oil 

 of cloves, is as high as 1,535; but I have alfo purchafed oil 

 by this name, which did not exceed 1,498, and which had 

 probably been adulterated by fome lefs refractive oil. 

 It is applicable to For fuch purpofes, the refractive power of opaque fubftances 

 opa e bo ics. ma ^ f ten ^ e deferving of inquiry, which could not be learned 

 by any means at prefent in ufe. For, in the ufual mode, a 

 certain degree of tranfparency is abfolutely neccflary ; but, for 

 trial by contact, the mod perfect opacity does not occafion the 

 Ieaft impediment. 

 Inftance. Among other inftances in which I have taken advantage of 



this circumftance, I may mention a fubftance that had been 

 found in one of the iflands of the North Pacific Ocean, which, 

 to all outward appearance and by various trials, feemed to be 

 perfect bees-wax, although it is fuppofed that there are no bees 

 in the ifland from which it was brought. On placing it by the 

 fide of a piece of bees-wax, in contact with a prifm, the perfect 

 equality of their refractive powers afforded a ftrong confirma- 

 tion of the opinion before formed of their identity. 

 Bodies of vary- For the examination alfo of media of which the refractive 

 mg denfity may denlity is not uniform, the general method of trial by deviation 

 jt, wholly fails ; on the contrary, by placing a varied medium in 



contact with a prifm, all its gradations of denfity, from greateft 

 to leaft, become at once the object of mere inflection. An in- 

 ftance of this may very readily be feen with a piece of gum, 

 the furface of which has been moiftened for a few minutes ; 

 when, by clofe application to a prifm, a refractive power may 

 be difcerned, varying from that of the water on the furface, 

 1,336, to nearly 1,51, the refractive power of gum arabic. 

 particularly the I fhould not fo much infill on this advantage, were it not 

 cryftalhne lens. f or t j ie opportunity hereby afforded of examining the cryftalline 

 lens of the eye, which is now known to be generally more denfe 

 in the centre than at its furface. 



Mr. Haukfbee, who was not aware of this difference, has 

 eftimated the refractive power of the cr) ftalline lens, by forming 

 it into a wedge by plates of glafs, fomewhat higher than I find 

 it to be ; but, with his accuftomed accuracy, he remarked the 

 apparent enlargement of an object, occafioned by the variation* 

 of its denfity, which he was unable to explain. 



In the table that follows, I have fet down, not only the limits 



of 



