zgS Dr. Herschei/s Experiments for investigating 
sical separation of the rays should actually take place, is owing 
to that particular circumstance in which, we have remarked, 
the lens differs from a prism, namely, the curvature of the 
refracting surface ; for although it has been proved that the 
figure of the first surface of a prism is not concerned in the 
formation of the blue bow, yet that of the surface through 
which it is seen by the eye is of material consequence, as will 
appear by the following experiment. 
An equilateral prism, one side of which I had made cylin- 
drical, was exposed so as to receive the incident light through 
the convex surface. In this situation, the eye being about three 
or four inches from the prism, a bow was formed which in 
every respect was like one I saw in another equilateral prism, 
whose three sides were flat ; but when the convexity of the 
first prism was turned towards the eye, the bow could no 
longer be seen, although the critical separation of the rays 
would undoubtedly form it in this, as well as in the other 
prism ; the two sides and angles of each exposed to the light 
being perfectly equal. By much attention to what may be 
perceived when the eye is placed at various distances, I found 
that the curvature of the surface through which I tried to see 
the bow, produced a focal contraction and subsequent inver- 
sion of the rays in their passage to the eye, and thus occa- 
sioned a total change of appearances. Now, since a ring or 
bow would not be visible in a prism bent round, if the side 
through which it must be seen were curved, we cannot expect 
to see such appearances in a lens, which every where presents 
us with a spherical surface. 
The effect upon the appearance of the bows, produced by 
the surface through which the rays must pass to come to the 
