[ 178 ] 
I. 
When light paffes from air into glafs, and the an- 
gle of incidence is next to 90°, whofe fine is unity > 
The fine of the refradion of the red \ 0 ' " , 
rays = 4° is .6493508 = fin. - -| 4 2<? 33,0 
And of the violet 44 = . 64 1 0256 = fin. 39 52 6 
Whofe difference o 37 27, 6 
is the greatefl angle at which the violet and red rays 
can diverge in the refradion from air into glafs, want- 
ing very little of 37 / i r . 
And when an unrefraded pencil paffes from glafs 
into air, as foon as the angle of incidence exceeds 
39 0 52' 6", the violet rays will begin to be refleded; 
and when the incidence exceeds 40° 29' 33", 6 the 
rays will be totally refleded. 
II. 
From Air into Water. 
The fine of refradion of the red is I* g ' " 
• 75 I 79 ° 5 =M 4 
Of the violet .7454080 = s. 48 11 39 
And the greateft divergence 0 33 5 
the angle of beginning refledion from water into air 
being 48° 1 1' 39". 
III. From 
