C 779 1 
iC fequently to one another, being given, to deter- 
u mine the proportion of their velocities at the time 
<( of their incidence on the firft plane.’* 
But as the inveftigation of the curve defcribed by 
the rays of light, in any hypothecs of attractive 
power, has been publifhed long ago (at lead by me 
in 1738), and by fuch a method, as leads to the fo-r 
lution of Mr. Melvil’s problem, I do not doubt but 
if he had feen that method, he would have refolved 
the problem, which he propofes, and perceived what a 
confiderable difference there is between the proportion 
of the velocities, and that of the fines of refraCtion. 
Mr. de Courtivron, who has made ufe of my fo- 
lution, is arrived at the following refult : 
If p denotes the ratio of the fines of incidence to 
the fine of refraCtion for one of the colours, and q 
1 1 
the fame ratio for any other, —y===- to ~7 =~^= 
J Vi —pp Vi —qq 
will exprefs the ratio, which the velocity of the firft 
rays bears to the velocity of the others. 
Now, in order to make ufe of fuch a theorem, 
if p and q are made equal to 7-5- and -f§, which are 
the proportions between the fines of incidence and 
refradion for the red and violet rays, the ratio of 
the velocities fought will come out in even num- 
bers, that of 45 to 44, which differs entirely from 
Mr. Melvil’s. 
Thus, if Mr. Short’s obfervations have led him to 
conclude, from Mr. Melvil’s principles, that the dif- 
ference of refrangibility cannot be caufed by the dif- 
ference of velocities (when the motion of light is 
performed in the manner of a projectile)* how furer 
f F 2 may 
