REFRJNGIBILirr of-LIGRT. 23 



numbers four, five, fix ; and each of them will be deflected 

 towards the perpendicular, in a degree proportioned to the force 

 acting on it. ; 



Fig. 5. reprefents the two prifms in contact, and the three rays 

 entering the glafs prifm perpendicularly, and emerging perpen- 

 dicularly from the fluid ; fo that the only refraction they fuffer 

 in their paffage, is in the confine of the two mediums. 



At the point of contact, the rays will be a&ed on by both 

 mediums, with the fame forces which they exerted when fepa- 

 rate. But thefe forces will act in oppofition, and therefore the 

 rays will only be affected by their difference ', and as the dif- 

 ference of attraction of the two mediums is the fame in all the 

 rays, they will all be equally refracted. The red ray is attract- 

 ed towards the glafs by the difference between the forces four 

 and five, the green by the difference between five and fix, and the 

 violet by the difference between fix and feven, each of which 

 differences is equal to unity, as reprefented in the figure. 



If the difperfive power of the fluid, contained in the prifma- 

 tic veffel, be diminifhed by decreafing the proportion of mer- 

 cury or antimony which it contains, the violet ray will begin 

 to be more refracted, and the red ray lefs refracted, than the 

 green ray. But if that quality be increafed, the contrary of 

 this will happen ; the red ray now becoming the molt refrangi- 

 ble, and the violet ray the leaft refrangible. 



If the difperfive medium employed, be of that precife degree 

 of ftrength, which enables it to refract the green ray in the 

 fame degree in which it is refracted by crown-glafs ; in this 

 cafe it has been afferted, that when light paffes obliquely from 

 the one of thefe mediums into the other, the green rays will 

 fuffer no refraction, but the red rays will be refracted from the 

 perpendicular, and the violet rays towards the perpendicular. 

 The reafon of this will appear from infpecting the three follow- 

 ing diagrams, 



Fig* 



