Min hn? =. =< 
4 
REFRANGIBILITY ofLIGHT. 23 
numbers four, five, fix; and each of them will be deflected 
towards the perpendicular, ina ‘aig proportioned to the force 
acling on it. 
Fic. 5. reprefents the two prifms in conta¢t, 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 paflage, is in the confine of the two mediums. 
Ar the point of contact, the rays will be acted 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. 
Ir 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 moft refrangi- 
_ble, and the violet ray the leaft refrangible. 
Ir the difperfive medium employed, be of that precife degree 
ef 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 appeals from infpecting the three follow- 
ing gus 
Fic. 
