22 On the UNEQUAL 



If 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 refraction in palling ob- 

 liquely from the one medium into the other at the point G, but 

 the violet ray will be refracted 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. 



This 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. 



These varieties of refraction will poflibly be better compre- 

 hended by the afliftance of diagrams. 



Fig. 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 paiUng from the glafs into 

 air, be refracted 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 mod 

 deflected, the green next, and the red ray lead. 



Fig. 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 feverai rays, are reprefented by the 



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