C 265 ] 
red light as 79763 to 78000. As light moves from 
the fun to us, by Dr. Bradley’s accurate eftimation, 
in 8' 12" (a), the pulfes of the asthereal fluid muft 
be propagated thro’ the fame fpace in about 8' i' . 
7. Hence alfo may be determined, in known mea- 
fures, the diftance between two fucceeding ethereal 
pulfes for D = 
Cl— FI 
8. Upon the hypothefls of the different velocities 
of different colours, we may underhand, at leaft in 
general, the reafon of the ft range analogy, difeover’d 
by Sir Ifaac, between the intervals of the fits, and 
the fpaces occupied by the feveral colours in the 
fpedtrum (a thing hitherto unexplained (b ) ; fince,from 
the velocities of the feveral rays, upon which depend 
the intervals of the fits, as has been now explained, 
arife likewife their feveral degrees of refrangibility. 
9. But, as it is of great confequence in philofophy, 
to diftinguilh between fadts and hypothefes, however 
plaufible, I obferve, that the various refrangibility, 
reflexibility, and inflexibility, of the different colours, 
and their alternate difpofitions, at equal intervals, to 
be reflected and tranfmitted, which are the whole 
ground-work of the Newtonian fyftem, are to be 
confider’d as undoubted fadts, deduced from experi- 
ment ; but that the velocities of different rays are dif- 
ferent in the manner now deferib’d, is no more than 
probable conjedture : and tho’ this point ftiould be 
decided, by a method that we are now to propofe. 
( a ) Eames’s Abridg. Tranfaft. Vol. VI. p. 157. 
(b) Compare Newt. Opt. Book I. Part 2. Prop 3. with Book 
II. Part 3. Prop. 1 6. ^ 
