f,j6 Dr. Herschel's Experiments for investigating 
this criterion I may now proceed to a review of more compli- 
cated phenomena. 
45. Explanation of various Appearances relating to prismatic 
Bows. 
If, in the open air, we look into the zenith with a right 
angled prism held across the eyes, we shall see two red bows 
convex towards each other. They are caused by the bright 
transmissions of the light of the heavens through the sides in 
which the bows appear ; for when to either of these sides the 
criterion of the plain glass is applied, we shall have coloured 
streaks. The course of the rays which produce the two bows is 
delineated in Fig. 3, Plate XII. ABC represents the prism, and 
the rays that can enter the eye when they fall on A B within 
the limits a b A from o° o' o" to 5° 56' 50", 5, which are the red, 
orange, yellow, and the brightest part of the green, will form 
the red bow ; and the situation of the eye at E will be had by 
the mean refrangibility of the rays which give the bow ; for 
as the angle Bed must be 49°s8' 19", 5, we have the obliquity 
B dc = 85° 2 1' 40", 5 and the angle C^E that conveys the ray 
to the eye will be 82° 49' 34",2. The same thing will happen 
on the other side of the prism, where the rays m n op q will 
come to the eye at E, in an equal but differently directed angle 
B q E, and cause an inverted red bow to be seen in the side 
AC. 
When we look down into the side of an equilateral prism 
we see a blue bow, but on lifting the eye and prism gently 
up together towards the zenith, the bow, at a certain altitude, 
will be changed from blue to red ; and by the application of 
the criterion, it is proved that we see the first by reflection, 
