C 262 ] . 
in part, before the Royal Society, or to make what 
ufe of it you pleafe. I am, 
S I R, 
enC *753- 6 * ** Yo ur m °ft obedient humble fervant, 
T. Melvill. 
Concerning the Caufe of the different Refran- 
gibility of the Rays of Light, by Mr . T. 
Melvill. 
Read March s, j . ^ N order to account for the diffe- 
JL rent refrangibility of the different- 
ly-colour’d rays, Sir Ifaac Newton •f', and feveral of 
his followers, have fuppofed, that their particles are 
of different magnitudes or denfities; but if there be 
any analogy between the refractive power and gra- 
vity, it will produce equal velocities in all particles, 
whatever their magnitude or denfity be ; and fo all 
forts of rays would be equally bent from their right- 
lined direction. 
2 . It feems therefore a more probable opinion, 
which others have advanced, that the differently- 
colour’d rays are projected with different velocities 
from the luminous body ; the red with the greateft, 
violet with the leaft, and the intermediate colours 
with intermediate degrees of velocity; for, on this 
hypothefis it is manifeft, that they will be differently 
refraCted 
'f Newton’s Optics, Qpery 29. 
