24 On the UNEQUAL 



Fig. 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. 



Fig. 7. reprefents a prifmatic veffel filled with butter of an- 

 timony, whofe mean refracting 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 ^ake 

 of round numbers) with the forces four and eight. 



Fig. 8. reprefents the two prifms in contact:, and confequent- 

 ly acting 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. 



But as the red ray is attracted 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. 



The 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 have tried thefe feveral cafes of refraction like wife with 

 compound object-glafTes, which ihew the effect better than the 

 prifms. Thus, if a plano-convex lens have its plane fide turn- 

 ed 



