[ 77 8 ] 
afford 'the means of diftinguifhing the difference of 
velocities between the rays of feveral colours. 
In fad:, if, according to that hypothecs, the red 
rays were fwifter than the others, it poflibly might 
happen, that the fatellite would appear of a reddifh 
colour in the beginning of the emerlion; I mean 
before the full time required for the whole tranf- 
mifiion of light from the fatellite to us. 
As to the examination of the number of feconds 
between the propagation of the red and violet rays, 
the two authors differ widely; and I cannot help 
afferting, that Mr. de Courtivron’s calculations are 
more furely grounded than the other’s. 
Mr. Melvil fuppofes, that the difference of vtlo- 
city between two forts of rays mud: be very near the 
difference of their fines of refradion, where their 
lines of incidence are the fame. From whence he 
concludes, that, as the fine of refradion of the red 
rays is about yj greater than the fine of refradion of 
the violet ones, the velocity of the firfl rays muff 
alfo exceed the velocity of the fecond by about yy. 
He indeed gives thofe proportions as only being 
nearly the fame ; for, fays he farther, to know ex- 
adly the ratio of the velocities from the fines of re- 
fradion, the following problem fhould be refolved, 
which he propofes to the learned : 
“ If two bodies fall, in equal angles of incidence, 
“ on a fpace terminated by parallel planes, in which 
“ any power ads perpendicularly to the planes (ac- 
<£ cording to the hypothecs in prop. 94.. lib. 1. of 
“ the Principia ), the ratio of the fines of the emer- 
4< gence to the common fine of incidence, and con- 
fequently 
