a MitLcils tf diUrmlnlng the R/fraclive and 



To afcertain the mean refrafkive and difpcrrive qualities of fluids, tlie Do£lor made u(« 

 of two kinds of apparatus. The firft, intended to afford a grofs knowledge of their pro- 

 perties, ccn'fitled of an apparatus of prifms. In the fecond method, where the fluids pro- 

 mifed to be of praflical ufe in Optics, they were more critically examined by means of Icnfcs, 

 in which tlie efieO, from being magnified, is rendered more confpicuous. 



The prifmatic apparatus confifts of a fmall, equiangular, thrce-fidcd prifm, of brafs. 

 Through this prifm, and parallel to one of its fides, are bored two holes, at a fmnll dif- 

 taneeJroin each other, equal in fize to the pupil of the eye. The fides of the piifm are 

 ground fl.\t, and there are two pieces of glafs with parallel fides, of the fame dimenfions as 

 the fides of the priOn. There are alfo prifms of the fame fize, and with the fame angles, of 

 different kinds of glafs ; and feme crown-glafs prifms with fmaller angles, which, by being 

 applied to the large pril'm, or to each other, vary the refracting angle at pleafure. 



When it is propofcd to try the properties of any fluid, one of the fmall plates of glafs is 

 applied over the holes on the fide of the brafs prifm. A few drops of the fluid are then 

 dropped into the hole, and, when it is full, the other phite is laid over the holes upon^ 

 the oppofitc fide, and the whole is fecured by tying a piece of packthread round the ends. 

 One of the glafs prlfri:s is nt>w to be applied to the brafs prifm contiguous with one of the 

 . parallel plates, the refracting angles of the two prifms being placed in oppofite directions fo, 

 as to form a fmall patallclopiped. 



Nothing farther is neceflary than to apply the eye to the hole which contains the fluid, 

 in fuch a wav as to obferve through it any bright well-defined objeft. The bars of the 

 . window anl'wer the purpofe verj- well in the day-time, and the moon or a candle in the 

 night. The intention of the two holes is for the fake of greater expedition. T3ic proper- 

 ties of two flui<ls may thus be e.\amined and compared at the fame time. As the prifma- 

 tic portion of fluid and the glafs prifm have equal refratliiig angles, and refract in oppo- 

 Ction to each other, it will eafily be underitood, that, if the object feen through tlie two 

 prifms coincides with the fame obje£t feen direttly, the mean refrafllve denfity of both 

 mediums will be the fame. When this is the cafe, if the object feen through thefc prifms 

 appears free from prifmatic colour, the difperfive power of the fluid medium is alfo the fame 

 with the difperfive powci of tlie glafs prifm. But otherwife they will be different. 



Thofe mediums, it is to be obferved, are faid to have the fame mean refractive denfity, 

 which, under equal obliquities of incidence, equally rcfraft the mean refrangible rays ; and 

 two nitdiums are faid to have the fame difperfive power, which produce an equal inclina- 

 tion of rays of the fame colour to the mean refrangible ray, when the whole rcfraition of 

 the mean refrangible ray is equal in both. 



When an obje(Et feen through the equal wedges of glafs and fluid appears coloured, one 

 of the fmaller glafs wedges is .to be applied, and fliifted, till the object appears colourlcfs. 

 It is eafy to diftinguifli, by the order in which the prifmatic colour lies, whether the fmall 

 prifm is to be applied in fuch a way as to increafc tne difpcrCon of the rays occafioned 

 by the fluid fo as to enable it to counterbalance that of the glafs, or whether the refracting 

 angle of the glafs prifm requires to be enlarged, to enable it to counteract the difperfiou 

 uccafioned by the fluid. 



By proceeding in this way to fliift the angles of the prifms till, firft, the dire£t and re- 

 fraCted images of an objeCt coincide, without regarding the colour, and, next, till the re- 



fradcd 



