[ 1 9 i ] 
Whence it appears, that although the differences of 
fines above fpecified, or the exceffes in Sir Ifaac’s the- 
orem, may, in refra&ions from different media into 
the fame rarer medium, be made equal, it does by 
no means follow, that the divergences of the feveral 
forts of rays (or if you chufe to call it their difper- 
fion) will be the fame in the two refractions; for Sir 
Ifaac’s exceffes 27, 2 74., &c. are the exceffes of fines; 
not of angles, as fome opticians feem to have mil- 
apprehended. 
Again, let an unrefracted pencil of light fill from 
common glafs into the air (Fig. 7.) at the incidence 
39°, and the angles of refraction will be, 
° / // 
Of the violet - - - - 79 2 - 2 
Of the red 75 43 55 
And their difference - 3 iH 7 is the diver- 
gence of the extreme rays. 
And the angle of refraction of the mean ray is 
77 0 16 ' 19". — By mean ray is underftood the ray 
whofe fine of refraction is a geometrical mean be- 
tween the fines of refraction of the extreme rays, 
the common radius being unity. 
Let now the fame rays be refracted the contrary 
way by a furface of water WT, then, to make the 
mean emergent ray parallel to the incident pencil, its 
angle of incidence mull be 86' ^ \ ex ~ 
treme rays will now converge at an angle of 2oi. 
minutes, nearly. 
Through the point of convergence <?, draw (by the 
Lemma) a plane to terminate the water, and 
unite all the rays into a colourlefs pencil os : and this 
emergent 
