46 ESSAYS anp OBSERVATIONS. 
incidence; thefe velocities muft be, to 
one another, nearly as their fines of re- 
fraction. 
39. Sir Ifaac found their fines of re- 
fraction from glafs into air, beginning 
from the extreme violet, to be* as 78, 772, 
T7935 772) 773) 175s 7722773 the fine of 
incidence being 50: From whence their 
fines of refraction out of air into glafs, 
beginning from the extreme red, and en- 
ding with the extreme violet, are tound 
to be as f. 78000, 77573, 77797, 77063, 
77496, 77330, 77220, 77000; the fine of 
incidence being 120120. Thefe numbers, 
therefore, nearly exprefs the velocities in 
air, of the feveral rays, before their inci- 
dence {. 
HENCE 
* Newt. Opt. book 1, part. 2, prop. 3. 
+ The extreme fines are plainly reciprocal to the fors 
mer, and thofe of intermediate colours are fourth pro~ 
portionals to the fine in Sir Ifaac’s experiment, 77. and 
78. 
+ The quantities which give the accurate proportion 
of the velocities, before incidence, muft be in a conftant 
ratioto the fines of refraétion, by which the above calcu- 
lations are made, have this condition : But, it is other- 
ways 
