the Cause of coloured concentric Rings. 283 
ing power of at least six or seven reiterated interior reflections, 
all of which may be seen through the side of the prism, had 
any share in the production of these streaks, I fixed on one 
side of it a glass, of which the lowest surface was emeried, 
and on the other a metalline plain mirror, but found that the 
streaks were both in number and colour perfectly alike in 
them all. 
By this account it is evident that the streaks derived from 
the blue bow contain not only the colours of the blue reflected, 
but also those of the red transmitted part of the spectrum. 
This fact is a clear indication of the office which is performed 
by the surface of the subjacent plain glass, which is simply 
that of reflecting back the rays of the transmitted red part of 
the spectrum, which being mixed with the blue part, both 
together, by their intersections, produce the observed streaks, 
as will be explained hereafter. 
That the colours of the transmitted part of the spectrum are 
reflected back into the prism, is a point which I suppose will 
be admitted ; but if it should be imagined that the red rays in 
the streaks of the blue bow might come into the prism by a 
scattered reflection of the light which falls on the plain glass 
under its base, then I say that a sheet of white paper or double 
emeried glass, ought to give the brightest streaks ; whereas, 
on the contrary, neither of them produces any ;* it is therefore 
evident, that a regular reflecting surface is necessary to their 
formation ; but such a surface, be it glass or metal, can only 
reflect red rays when it receives them ; and since we know 
that the red part of the spectrum is transmitted, and must fall 
on the reflecting surface, it is but fair to conclude that the 
* See the last paragraph of the preceding article. 
