e8o Dr. Herschel's Experiments for investigating 
4 ,6. The first Surface of a Prism is not concerned in the Formation, 
of the blue Bow, nor of the Streaks that are produced by a plain 
Glass applied to the efficient Surface . 
It has already been mentioned that the bows are formed by 
scattered light ; but to have a direct experimental proof that 
such light, if not absolutely necessary to the formation of the 
bows, is at least equally efficient with regularly refracted light, 
I took a prism with one side of it roughened on emery, and 
receiving the light through it when the eye was in the situa- 
tion required for seeing the blue bow, I saw it as completely 
formed by scattered light, as it could have been by light regu- 
larly refracted through a polished side. 
A natural consequence of this experiment seems to be, that 
the form of the surface through which light enters can be of 
no consequence; this will however admit of a more convincing 
proof, as follows : upon the middle of the side of a right 
angled prism, through which the rays entered that caused the 
blue bow, I laid a piano convex lens of an inch and a half 
focus ; the result was, that not the least alteration could be 
perceived eitherrin the form or in the colour of the bow, both 
which remained as perfect under the place where the incident 
rays passed through the lens as they were on each side of it. 
When I changed the convex lens for a plano-concave glass 
of the same focus, appearances were still the same ; and when 
by a critical application of a plain glass I produced coloured 
streaks from the base of the prism, the interposition of either 
the convex or concave glass was equally immaterial. A 
scattering glass applied to the incident ray, had no other effect 
than to diminish the brightness of the bow. 
