22 On ith UNEQUAL 
Ir the mean refractive denfity of the difperfive fluid, con- 
tained in the prifm BCD, be fo far increafed as to become 
equal to the mean refractive denfity of the glafs prifm ABC, 
the mean refrangible ray will fuffer no refraétion in pafling ob- 
liquely from the one medium into the other at the point G, but 
the violet ray will be refra€ted towards the perpendicular, and 
the red ray will be refracted from the perpendicular. The rea- 
fon of which is, that the difperfive medium refracts the violet 
ray more, and the red ray lefs than the other medium; fo that 
the former may be confidered as an equally denfe medium with 
the latter relative to the green ray, but more denfe relative to 
the violet ray, and lefs denfe relative to the red ray. 
Turis cafe of refraction takes place in the confine of crown- 
glafs and butter of antimony, when the latter is fo far diluted as 
to have the fame mean refractive denfity with crown-glafs ; 
that is to fay, when both mediums equally refract the green ray, 
under equal obliquities of incidence. 
THeEse varieties of refraction will poflibly be better compre- 
hended by the affiftance of diagrams. 
Fic. 3. reprefents a prifm of crown-glafs, which is entered 
perpendicularly by a red, green and violet ray, moving parallel 
with each other. As their incidence on the fecond furface of 
the prifm is oblique, they will, in pafling from the glafs into 
air, be refraGted from the perpendicular. This deflection of 
the light from its rectilineal courfe, is fuppofed to be produced 
by the perpendicular attracting forces, reprefented by the num- 
bers five, fix and feven. The violet ray will therefore be moft 
deflected, the green next, and the red ray leaft. 
Fic. 4. reprefents a prifm of difperfive fluid, which the 
three rays enter with the fame degree of obliquity which they 
had before they emerged from the glafs prifm. The attracting 
forces of the fluid for the feveral rays, are reprefented by the 
numbers 
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