REFRANGIBILITY of LIGHT. Ps 
place of crown-glafs, and the advantage propofed being a faving 
of the light loft by refleGtion. 
THE experiments of Mr DoLtonp 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 refra¢tive denfity of fluids, from experiments made both 
before the difcovery of DoLLOND and fince. But though fome 
of the fluids examined were poffeffed of the difperfive quality 
im a remarkable degree, this is pafled over unobferved, and it 
would feem unfufpected, if we except the very ingenious con- 
jecture of Mr Micuext ; 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 
DottonD, 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 Micuet fufpected that lead, even in this form, might 
increafe the diflipative refraGtion, as it does in the compofition 
of glafs. The refult of his experiments on this fubject may 
be feen in the additions to Dr PriestTxLey’s Optics, at the end 
of the fecond volume; 
Of the methods employed for inveftigating 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 praétical part of 
optics, were more critically examined by means of lenfes, where 
Vor. II. B the 
