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refraCted in the prifmatic order, according to obfer- 
vation. 
3. On luppofition, that the different refrangibility 
of light arifes folely from the different velocities of 
the rays before incidence, thefe velocities muff be to 
one another nearly as their lines of refraction. I 
fay nearly 3 for their exaCt proportion cannot be dif- 
covered, but by the folution of the following pro- 
blem, which I take this opportunity of propofing to 
the learned: 
If two bodies fall, in equal angles of incidence, on 
a fpace terminated by parallel planes, in which 
any power adts perpendicularly to the planes 
(according to the hypothecs in Prop. 94, Lib. I. * 
of the Principia ) the ratio of the fines of emer- 
gence to the common line of incidence, and 
confequently to one another, being given, to 
determine the proportion of their velocities at 
the time of their incidence on the firft plane. 
4. Their velocities in any given medium (fuppofe 
air) being once determined, their velocities in any 
other may be ealily difcover’d ; for they are to thofe 
in air as the fine of incidence to the fine of refraction, 
when the ray paffes from air into the other me- 
dium -f*. 
5. While the differently-colour’d rays are fuppofed 
' to move with one common velocity, any pulfes, ex- 
cited in the acthereal medium, muff overtake them at 
equal 
f Principia, Phil. Nat, Prop. 95, Lib, I. 
