[ j8 4 ] 
of the plane of refraction with the fide of the prifm 
that is required. 
Thus the angle SOB being 30°, VOR will be 
18' 12% VOG= 5 o° 3 8' 4% ROG==5o° 19' s *” i - 
whofe hnes are as the fines of refraction of the violet 
and the red, in palling from glafs into water at a 
common angle of incidence. And therefore, the an- 
gles of the emergence of the rays OV, OR, in pair- 
ing from water into glafs will be equal, that is Vv 
will in its paflage through the glafs prifm, be parallel 
to Rr, and the rays meeting with equal and contra- 
ry refractions at the points v, r, 0 , as they fuffered 
at V, R, O, will emerge colourlefs at 0. 
Yet we muft not be furprized if the pencil os is 
not abfolutely pure light (even fuppofmg, the mat- 
ter, the figure, and the dilpofition, of the media to 
be faultlefs) becaufe (i°) perhaps the refraCtive powers 
have not been determined with fufficient exaCtnefs 
(2 0 ). If the glafs plate which contains the water 
be not very thin, the light will have received a flight 
tinCture in pafling through it at O : This however 
may be remedied by confining the water between two 
glafs prifms. And (3 0 ) it its fcarce poffible to make 
experiments of this kind with a pencil of light fo 
flender as the theory prefcribes (fee § 2.) 
But proper allowances being made on thefe ac- 
counts, and the refraCting planes adjufted as the lem- 
ma directs, the light will emerge fufficiently pure to 
juftify the theory. And the refractions of either me- 
dium being given, it will appear from the experi- 
ments whether thofe of the other medium have been 
determined with fufficient accuracy. 
Obfervc 
