24 On the UNEQUAL 
Fic. 6. reprefents a prifm of crown-glafs, in which the red, 
green and violet rays, at their emergence into air, are attracted, 
as before, with the forces five, fix and feven. 
Fic. 7. reprefents a prifmatic veffel filled with butter of an- 
timony, whofe mean refraGting force is equal to that of the 
crown-glafs, fo that the green ray is attracted by it with the 
force fix. But in confequence of its great difperfive power, 
the red and violet are attracted, (we fhall fuppofe for the fake 
of round numbers) with the forces four and eight. 
Fic. 8. reprefents the two prifms in contaét, and confequent- 
ly a@ting in oppofition to each other. Now, the force with 
which each of the mediums acts on the green ray, is reprefented 
by fix; the difference between which being nothing, the green 
ray will proceed in its rectilineal courfe, as it would do in the 
fame uniform medium. 
Bur as the red ray is attraéted by the crown-glafs with a 
force reprefented by five, and by the difperfive medium with a 
force equal only to four, it will, in paffing out of the former 
into the latter, be deflected towards the crown-glafs, by the dif- 
ference between thefe forces, which is equal to unity. 
Tue violet ray, on the contrary, is attracted by the crown- 
glafs with the force feven, and by the difperfive medium with 
the force eight, and will therefore be refracted towards the lat- 
ter, in the fame degree in which the red ray is refracted from 
it, as reprefented in the figure. It is'a circumftance worth re- 
marking, that a particle of red light, and a particle of violet 
light, under precifely the fame circumftances of expofure to 
the action of grofs bodies, fhould be urged in contrary direc- 
tions. 
I nAve tried thefe feveral cafes of refraction likewife with 
compound objedt-glaffes, which fhew the effect better than the 
prifms. Thus, if a plano-convex lens have its plane fide turn- 
ed 
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