the Cause of coloured concentric Pangs. 297 
eye, may be still better examined by laying the plain side of 
a plano-convex glass of a short focus upon the flat side of a 
prism, through which we see either of the bows ; for when 
the eye is near the focus of the lens, they will be entirely ef- 
faced as far as they are covered by the lens. 
A consequence of great importance may be drawn from 
these experiments ; for since the cause of the coloured ap- 
pearances, which have been called bows when seen in a prism, 
is now perfectly understood to be the critical separation of the 
colours of the incident light, it must be admitted that such a 
separation will certainly take place whenever a beam of light 
can find an entrance into glass, so as to make the required 
angles either with an interior or exterior surface, be it in the 
shape of a prism, lens, or solid of any kind, although the 
figure of the last transmitting surface should not permit such 
coloured-appearance-making-rays to reach the eye. A plano- 
convex lens will consequently by its construction separate the 
rays of light which enter at the convex surface in such a 
manner, as by reflection to produce what, if it could be seen, 
would be called a blue bow, and by rays that come in at the 
plain side, separate them by intromission so as to produce a 
red one. 
To remove all doubt about the truth of this theory, I ground 
a small part of a plano-convex lens flat, that I might look into 
it, as it were, through a window, to see what passed within. 
The flat made an angle with the base of about thirty-four de- 
grees, and I saw through it very plainly, in different directions 
of the illumination, a blue bow by light entering at the convex 
surface, and a red bow by light coming in at the plain one. 
With regard to a plain glass contained between parallel 
