1 64^ Dr. Herschel's Experiments for Investigating 
LIX. Experiments on the multiplying Power of Surfaces in con- 
tact , which modify the Form of prismatic Appearances. 
The simplicity of the following experiments is such as will 
ensure them a ready admittance, even by those who may not 
have an opportunity of repeating them ; their application also 
to some of the most intricate phenomena of modifying the 
form of the prismatic appearances, must render them of con- 
siderable value. 
First experiment. Upon a plain metalline mirror 52 inches 
long, and 4 broad, I laid the base A B C D, figure 11, of a 
right-angled prism ; and having darkened the room, a candle 
was placed so as to throw its light upon one side of the prism, 
the reflection of which from the base, I saw through the other. 
The eye was then gradually lifted up to such an altitude, that 
a blue bow, if it were made visible by the admission of an 
uniform scattered light, would extend from a to b; the candle 
was then withdrawn, till only the inverted flame of it remained 
visible at c. A small pasteboard screen as long as the prism, 
and bent at the top as in figure <9, must be hung by the end 
a upon the vertex of the prism, to cover the reflected image 
of the candle, and the side b c must be short enough to leave 
about one or two tenths of an inch open for scattered light to 
enter', so as by reflection from the mirror, in proper angles 
to make the blue bow visible. Every thing being in this 
arrangement, and the room properly darkened, as well as 
the eye guarded from the direct light of the candle, place the 
pasteboard screen on the prism, and you will then perceive a 
very bright small spectrum of red and green light at d, which 
consists of those rays that in the blue bow place are 
