REFRANGIBILItr of LIGHT. 9 



place of crown-glafs, and the advantage propofed being a faving 

 of the light loft by reflection. 



The experiments of Mr Dollond proved, that the difper- 

 five power of water is lefs than that of the glafs with which he 

 made his experiments ; and it feems wonderful that this fhould 

 have been almoft the only attempt made to inveftigate this qua- 

 lity in fluid mediums. We find many tables afcertaining the 

 mean refractive denfity of fluids, from experiments made both 

 before the difcovery of Dollond and fince. But though fome 

 of the fluids examined were poflefled of the difperfive quality 

 in a remarkable degree, this is paflfed over unobferved, and it 

 would feem unfufpected, if we except the very ingenious con- 

 jecture of Mr Michel ; to whom it occurred, that the appa- 

 rent difference in the experiment above mentioned, made by Sir 

 Isaac Newton, from the fame experiment repeated by Mr 

 Dollond, might arife from the former ufing, inftead of pure 

 water, a folution of Saccharum Saturni, which he mentions 

 his having fometimes made ufe of to increafe the refraction. 

 Mr Michel fufpected that lead, even in this form, might 

 increafe the diflipative refraction, as it does in the compofition 

 of glafs. The refult of his experiments on this fubject may 

 be feen in the additions to Dr Priestley's Optics, at the end 

 of the fecond volume* 



Of the methods employed for invejligating the optical qualities of 

 different mediums. 



In afcertaining the mean refractive and difperfive qualities 

 of fluids, I made ufe of two kinds of apparatus. Where the 

 properties of the fluids were entirely unknown, prifms were 

 employed to come to a grofs knowledge of their properties, and 

 thofe fluids which promifed to be of ufe in the practical part of 

 optics, were more critically examined by means of lenfes, where 



Vol. III. B the 



