- 
my te 
PHYSICAL ann LITERARY. 47 
40. HENCE their velocities in any other 
medium may be found; for they are, to 
thefe, 
ways manifelt, that they give only a grofs approximation 
to the truth. From what follows, perhaps, an exaéter 
“computation might be made, if a proper mean angle of 
incidence were made ufe of, although the quantities in 
the canon are really not in a conftant ratio. 
TAB. iii, fige 1. Let two rays, talling in the fame line 
of incidence IC, with different velocities, upon AB the 
furface of a denfer pedir) be lvckeatted! nto different 
lines CR, CV. Taking any line CD in the perpendicu. 
Jar to reprefent the total action of the refracting power 
on the lefs refrangible ray, and CE on the more refrangi- 
ble; If, through D and &, parallels to IC be drawn, 
meeting the refra@ted rays in V, Rand G; it is plain, 
that CR, CV will be as their refpective velocities after 
refraction ; and DR, EV, as their velocities before inci- 
dence. Since the whole acceleration which a given 
power produces in a body, is, ceteris paribus, as the 
time in which it operates ; CD muft be to CE nearly as 
the time which the {wifter ray takes to pafs through the 
refracting fpace, to that which the flower ray takes in 
paffing through the fame, inverfely, as their velocities 
before incidence; that is, as EV to DR: but CD is like- 
Ways to CEas DG to EV; therefore DR, EV, and DG, 
are continued proportionals; therefore DR is te EV in 
the fubduplicate ratio of DR to DG: But DRis to DG 
“ima ratiocompounded of DR to DC, and DC to DG, that 
* is, in the compounded ratio of S$, DCR to S, DRC, and 
of 
