the Cause of coloured concentric Rings. 2 6g 
experiments on prismatic bows succeed best, when the heavens 
are totally overcast with an uniform cloudiness. 
To analyze the production of this bow, let ABCDE, Fig. 2, 
Plate XII. be rays of light moving in air, in such directions as to 
Fall on the exterior base FG, of a piece of glass, upon the points 
u jQ y $ e ; then, if it be required that these rays, after their in- 
tromission into the glass should meet in the point H and form 
the red bow, the angles A«H, B (3 H, CyH, D £H, and Ee H, 
must be respectively equal to 130° 29' 33 ", 6 ; 133 0 40' 33^2 ; 
134 0 29' 28"s ; 135° 36' i3"a ; and 136° 10' 38", o ; from which 
we have the angles A/ 3 B, A7C, A^D, and AeE, which a 
red ray would make were it to pass out of glass into air, 
equal to 3 0 if 45", 5 ; 4 0 7' 30 "3 ; 5’ 19' 1 7" 5 5 and 5° 36' 50", 3. 
Now by the laws of the different refrangibility of light, the 
red rays are intromissible at a, when by refraction they make 
the angle H«F = 49 0 30' 26'% ; but the orange cannot be in- 
tromitted any where between « and (3 with any effect on the 
red bow, since it is only at ( 3 , where the angle H (3 F is 49 0 33' 
12"3, that they can enter the glass so as to come to the eye 
at H. The yellow rays will, for the same reason, be efficiently 
intromitted only at y, where they will make the angle HyF 
49" 38' 2", 3, and the brightest half of the green rays will find 
an efficient entrance from $ to e, since the smallest angle of 
their intromission H 8 F is 49 0 43' 4", 3, and the angle H e F, 
which terminates the red bow, is 49 0 46' i2"5. The arrange- 
ment of the colours of this bow will be seen, as it was in the 
blue bow, from the letters placed above the base, which de- 
note those that are intromitted so as to come to the eye ; the 
rest of the colour-making rays, which cannot come in that 
direction, being marked by letters placed under the base. The 
mdcccix. N n 
