C i8 3 ] 
We have here fuppofed the ray SO to be homo-” 
geneous, of a mean refrangibility ; but if it is a ray 
from the Sun the image at j will be very much tinged. 
The colours will have been feparated at O ; a fmall 
matter more at M, but they will diverge very confi- 
derably at o ; for fetting afide the refractions at O 
and M ; that is, fuppofing a pencil Mo to pafs unre- 
fraded in water till it falls upon a furface of air at 
an angle of incidence of about 47° 32 / -i ) the diver- 
gence of the extreme rays will be about 2° 51 'L.: a 
fmall difference of fines anfwering to a confiderable 
difference of the angles when they approach to 90°: 
the ultimate difference to which they converge, being 
(from water into air) 7 0 26 'JL. 
12. Let a pencil of the lolar light SO (Fig. 6.) 
fall upon the furface of water B C, the extreme rays 
being refraded into OV, OR; it is required to af- 
fign the glafs prifm PN« (whofe fedion PN7Z is an 
ifofceles triangle) fuch, that the bafe N;z being paral- 
lel to SO, and the furface of the water AC being 
inclined to the bafe N n in the fame angle as the fur- 
face BC; the extreme rays, in their paflage through 
the glafs prifm, fhall be parallel ; and all the rays 
fhall emerge colourlefs in the line SO os-, that is, in 
the incident ray produced thro’ both the media. 
The angle SOB, and the refradions from air into 
water, being given, the angles VON, RON, and 
their difference VOR, are given. Draw therefore, 
by the lemma, the line OG, making the fine of ROG 
to that of VOG, as the fine of refradion of a red ray, 
in paffing from glafs into water, is to the fine of refradion 
of a violet ray, their angles of incidence being equal, 
and PN perpendicular to OG will be the interfedion 
