[ 734 ] 
differently refradted, and that light confifts of parti 
that differ in degrees of refrangibility. 
Every ray of light pafling from a rarer into a 
denfer medium, is refradted towards the perpendicu- 
lar ; but from a denfer into a rarer one, from the per- 
pendicular ; and the lines of the angles of incidence 
and refradtion are in a given ratio. But light con- 
fiding of parts, which are differently refrangible, 
each part of an original or compound ray has a ratio 
peculiar to itfelf ; and therefore the more a hetero- 
gene ray is refradted, the more will the colours di- 
verge, ffnce the ratios of the fines of the homogene 
rays are conftant j and equal refradtions produce 
equal divergencies. 
That this is the cafe when light is refradted by 
one given medium only, as fuppofe any particular 
fort of glafs, is out of all difpute, being indeed felf- 
evident ; but that the divergency of the colours will 
be the fame under equal refradtions, whatfoever me- 
diums the light may be refradted by, tho’ generally 
fuppofed, does not appear quite fo clearly. 
However, as no medium is known, which will 
refradt light without diverging the colours, and as 
difference of refrangibility feems thence to be a pro- 
perty inherent in light itfelf, Opticians have, upon 
that confideration, concluded, that equal refradtions 
muff produce equal divergencies in every fort of me- 
dium : whence it fhould alfo follow, that equal and 
contrary refradtions muff not only deffroy each other, 
but that the divergency of the colours from one re- 
fradtion would likewife be corredted by the other ; 
and there could be no poffibility of producing any 
fuch thing as refradtion, which would not be affedted 
by 
