REFRANGIBILITY of LIGHT. 21 
experiments have proved, that this fappofed general law of re- 
_ fraction does not hold in nature. 
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_In the inftance before us, if we fuppofe the force with which 
glafs attracts the red, green and violet rays to be reprefented by 
the numbers five, fix and feven ; then may the force with which 
the difperfive fluid attracts thefe rays, be reprefented by the 
numbers four, five and fix. For the reafon why all the rays are 
equally refracted in their tranfition from one of thefe mediums 
into the other, is becaufe the rare medium has the property of 
refraCting the violet rays more, and the red rays lefs, than the 
denfe medium, when the obliquity of incidence is fo propor- 
tioned to their denfity, that the mean refrangible ray fhall fuffer 
the fame refra¢tion in both. 
Now, in the cafe above ftated, the attraction of the rare 
medium for the feveral rays, is fo proportioned to the attraction 
of the denfe medium for thefe fame rays, that the difference of 
thefe attractions is every where equal, and confequently the re- 
fraction arifing from thefe differences of attraction is alfo equal. 
Thus the green ray is attratted by the denfe medium with the | 
force fix, and by the rare medium with,the force five, the dif- 
ference of which is ome; and there is the fame difference be- 
tween the attracting forces acting on the red and violet rays im 
the two mediums, being in one cafe the difference between five 
and four, and in the other between fix and feven; fo that the 
difference of attracting force, and confequently the refraction 
fuppofed to arife from it, is the fame in all the rays, being al- 
ways that which will be produced by an attracting force, repre+ 
fented by unity. aes 
Ir the difperfive power of the rare medium, of which the 
prifm BCD is formed, be ftill farther increafed, the red rays 
will become the moft refrangible, and the violet rays the leaft 
refrangible ; a law of refraCtion, which, as has been already 
explained, obtains when light is refracted in the confine of 
exown-glafs and oil of turpentine, and of many other fluids. 
Te 
