neers 
REFRANGIBILITY of LIGHT. 11 
two prifms coincides with the fame objet feen diretly, 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. 
TuoseE 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 refraftion of the mean 
refrangible ray is equal in both. 
WHEN an objett, 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 {mall 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 refra€ting angle of the glafs prifm requires to be enlarged, 
to enable it to countera¢t the difperfion occafioned by the fluid. 
By proceeding in this way to fhift the angles of the prifms, 
till, firft, 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- 
B2 fraction 
