the Cause of coloured concentric Rings. 277 
and the last by transmission. For, suppose a b, Fig. 4. Plate XII. 
to be a ray of a mean refrangibility between the violet, indigo, 
blue, and half the green ; when this falls on the side A C of the 
equilateral prism ABC with an obliquity ab A of 57 0 58' 28'', 5, 
it will be refracted so as to make the angle C cd 70° 2' 56", 8 
which gives 49 0 57' 3", 3 for the angle C dc ; and consequently 
the ray def E will come to the eye by the same angles of re- 
flection and refraction as it entered the prism, and make A/E 
equal to Aba. The eye at E will therefore see a blue bow. 
Then if a plain glass be applied to the transmitting side A C 
there can be no streaks ; for blue bows being caused by the 
critical separation of the rays occasioned by the Newtonian 
reflexibility, the plain glass must be in montact with the re- 
flecting side ; and as soon as we hold it against B C, the 
coloured streaks will make their appearance. The change of 
the colour of the bow, on lifting the prism and eye together 
towards the zenith, is represented in figure 5 ; for the light 
from the sky, which will enter the prism on the side A B, will 
eclipse the blue bow which was seen before by light entering 
from the ground through the side A C in figure 4 ; then if a b 
fig. 5 is a ray of the mean refrangibility of the red bow, i^ 
will by refraction give the angle Bed 49 0 38' 13", 5, from 
which we obtain Bdb equal to 70° 21/ 40/ 5, and the ray will, 
by a second refraction, come to the eye in an angle C e E of 
58° 44' 12", 4, where the red bow will be seen ; but in order 
to produce coloured streaks, the plain glass must now be ap- 
plied to the transmitting side A B. 
When a right angled prism is held in the hand, so that the 
light of the sky through an open window may fall upon the 
base, if then an observer with his back to the light looks through 
O o 
MDCCCIX 
