the Cause of Coloured Concentric Rings . igj 
instantly be changed into those of the red bow. The rays 
which cause them, enter through the side b c , are critically 
separated by intromission , and form the red bow, but are at the 
same time turned into streaks by the side b c, of the prism 
bed, and go to the eye at E. 
The experiment will succeed equally well, if, instead of the 
prism bed, a highly polished plain slip of glass of the size of 
the base of the prism a b c is tied to it. 
LVI. Illustration of the Dependence of Rings, seen in a Prism 
upon the critical Separation . 
If it should now be granted, that streaks which may be seen 
by applying a plain glass to the side of a prism, depend en- 
tirely upon critical separation, it may still be doubted, whether 
the rings which are produced, when a prism is laid upon a 
spherical surface, are likewise to be ascribed to the same 
cause, but this may also be decided by a very satisfactory 
experiment as follows. 
Upon a small board, lay a sheet of white paper to reflect 
light upwards, and through the paper fasten three short tacks 
into the board ; then place an object glass upon the tacks, and 
put a right angled prism across its surface, which should be 
of the convexity of a globe of about 30 or 40 feet diameter. 
A pasteboard screen, formed as in figure 9, must be hung 
over the vertex to darken the exposed side, that only the 
scattered light which comes from the paper, may enter the 
prism through the base, and cause a red bow. The board 
should be placed upon a stand, near a door which admits the 
unconfined light of the heavens, where no adventitious 
colours will disturb the experiment. As soon as the eye 
MDCCCX. Y 
