[ 182 3 
fame line FN. And if the medium be terminated 
on the other fide by any plane KN to which FN is 
perpendicular, the pencil N Y, continued in the air, 
will remain colourlefs. 
For inftance, if the medium GK is glafs, and the 
angle RF V is 14' 26", ZFR the angle of incidence 
of the red rays will be found of 17 0 54' 14"; and 
the angle of refradtion XFN, common to all the 
rays, will be 12° 6' 34' 
But if the plane HG, to which ZX is perpendi- 
cular, paffes not thro’ the point of reunion F, but on 
this or the other lide of it ; the rays in their paffage 
thro’ the medium, though parallel to each other, 
will be laterally feparated. 
11. Let a ray SOL (Fig. 5.) of a mean degree 
of refrangibility be refradted by AB the fide of a 
glafs prifm ABC, fo that the refradted ray O M may 
be perpendicular to the fide of the prifm AC; it is 
required to apply to this another prifm of a differently 
refradting fubftance, as of water, fo that the ray Mo 
being refradted at 0, by the fide DC, the refradted 
ray so may be parallel to OS. 
The angle of incidence SOP, and the refradtive 
power of the glafs being given, the angle SOM, 
and its fupplement LOM, are given produce Mo to n\ 
and becaufe os is to be parallel to LO take for the 
difference of the angles in the lemma, the given 
angle jios (=LOM), and through the point 0 draw 
ropy fo that the fine o i p on may be the fine of pos y 
as the fine of incidence to that of refradtion, when 
a meanly-refrangible ray paffes from water into air y 
and D 0 C, perpendicular to rp } will be the pofition 
of the fide required. 
1 
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