the Cause of coloured concentric Rings. 279 
one side upon a sheet of white paper placed in a window, with 
the base towards the observer, as represented in fig. 8. In 
this position, the light from without reflected by the paper 
under the prism will be brighter than that from within the 
room, and the very oblique incident rays ab will be refracted 
by the horizontal side A B, so as to make the angle Bed equal 
to 49 0 38' 19", 5, from which we have B dc — 85° si' 40", 5, 
and by refraction C<?E = 82° 49' 34"2, the eye placed at E 
will therefore see an erect red bow in the horizontal side A B, 
which may be drawn over into the perpendicular side without 
change of position ; for the scattered rays reflected from the 
paper will also enter the prism in the same oblique angle of 
incidence from the opposite direction a b fig. 9 ; where having 
caused the red bow by an intromissive critical separation at c, 
they will come to the eye after a common reflection from the 
side A C, in the same angle as before. 
When an inverted red bow is to be seen the eye must be 
placed a little lower, and the calculation of the angles in the 
10th and 11th figures, which represent the course of the rays, 
being similar though differently directed, will be sufficiently 
understood by an inspection of them ; but as in fig. 8 and 9, 
the intromissive separation was produced by the horizontal 
side, so it is, in these figures, effected by the vertical one ; all 
which may be proved by a proper application of the criterion. 
There are many other phenomena attending the bows, but 
as they are more intricate, and not necessary for my present 
purpose, I leave them to the ingenuity of those who have en- 
tered into the preceding calculations, which are quite sufficient 
to point out the method that should be taken for explaining 
them. 
O o 2 
