7h. On the UNEQUAL 
Tuus it happens to the red and violet rays, whichfoever 
clafs of difperfive mediums be employed. But the refrangibi-_ . 
lity of the intermediate orders of rays, and efpecially of the 
green rays, will be different when the clafs of difperfive medi- 
ums is changed. : 
Tuus in the firft cafe, where the red and violet rays are ren- 
dered equally refrangible, the green rays will emerge moft re- 
frangible, if the firft clafs of difperfive mediums is ufed, and 
leaft refrangible if the fecond clafs is ufed. And in the other 
two cafes, where the violet become leaft refrangible, and'the 
red moft refrangible, and where thefe two kinds of rays are re- 
fraéted in oppofite direGtions, the green rays will join the red, | 
if the firft clafs of difperfive mediums be employed, and will 
arrange themfelves with the violet, if the fecond clafs be made 
ufe of. 
ONLY one cafe more of unequal refrangibility remains to be 
ftated ; and that is, when light is refracted in the confine of - 
mediums belonging to the two different claffes of difperfive 
fluids. In its tranfition, for example, from an effential oil, or 
a metallic folution, into the muriatic acid, the refractive denfity 
of thefe fluids may be fo adjufted, that the red and violet rays 
fhall fuffer no refraction in paffing from the one into the other, 
how oblique foever their incidence be. But the green rays will 
then fuffer a confiderable refraction, and this refraction will be 
from the perpendicular, when light pafles from the muriatic 
acid into the effential oil, and towards the perpendicular, when 
it paffes from the effential oil into the muriatic acid. The other 
orders. of rays will fuffer fimilar refractions, which will be 
greateft in thofe adjoining the green, and will diminifh as they 
approach the deep red on the one hand, and the extreme violet 
on the other, where the refraction ceafes entirely. 
Tue manner of the produdtion of thefe effects, by the at- 
tration of the feveral mediums, may be thus explained. 
WE 
