REFRANGlBILITr of LIGHT. n 



two prifms coincides with the fame object feen directly, the 

 mean refractive denfity of both mediums will be the fame. 

 When this is the cafe, if the object feen through thefe prifms 

 appears free from prifmatic colour, the difperfive power of the 

 fluid medium is alfo the fame with the difperfive power of the 

 glafs prifm. But otherwife they will be. different. 



Those mediums, it is to be obferved, are faid to have the 

 fame mean refractive denfity, which, under equal obliquities of 

 incidence, equally refract the mean refrangible rays, and two 

 mediums are faid to have the fame difperfive power, which pro- 

 duce an equal inclination of rays of the fame colour, to the 

 mean refrangible ray, when the whole refraction of the mean 

 refrangible ray is equal in both. 



When an object, feen through the equal wedges of glafs and 

 fluid, appears coloured, one of the fmaller glafs wedges is to 

 be applied and fhifted till the object appears colourlefs. It is 

 eafy to diftinguifh, by the order in which the prifmatic colour 

 lies, whether the fmall prifm is to be applied in fuch a way as 

 to increafe the difperfion 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 difperfion occafioned by the fluid. 



By proceeding in this way to fhift the angles of the prifms, 

 till, firfb, the direct and refracted images of an object coincide, 

 without regarding the colour ; and, next, till the refracted 

 image appears colourlefs, without regarding the coincidence ; 

 the ratio of the mean refractive and difperfive powers of that 

 kind of fluid, and that kind of glafs, with which the experi- 

 ments are made, will be obtained, from the angles of the prifms 

 being given in both cafes. 



In order to afcertain the abfolute refractive denfity of glafs, 

 or any other medium, that is to fay, the general ratio of the 

 fines of the angles of incidence to the fines of the angles of re- 



B 2 fraction 



