214 E) r - Herschel’s Experiments jor investigating 
necessary. We may, for instance, convey light into the body 
of the subjacent glass through its first surface, and let it be 
reflected within the glass at a proper angle, so that it may 
come up through the point of contact, and reach the eye, 
having been transmitted through no more than three surfaces. 
To prove this 1 used a small box, blackened on the inside, and 
covered with a piece of black pasteboard, which had a hole 
of about half an inch in the middle. Over this hole I laid a slip 
of glass with a 56-inch lens upon it ; and viewed a set of rings 
given by this arrangement very obliquely, that the reflection 
from the slip of glass might be copious. Then guarding the 
point of contact between the lens and the slip of glass from 
the direct incident light, I saw the rings, after the colour of 
their center had been changed, by means of an internal reflec- 
tion from the lowest surface of the slip of glass ; by which it 
rose up through the point of contact, and formed the primary 
set of rings, without having been transmitted through the 
lowest surface of the subjacent glass. The number of trans- 
mitted surfaces is therefore by this experiment reduced to 
three ; but I shall soon have an opportunity of showing that 
so many are not required for the purpose of forming the rings, 
XXIV. Of the Action of the first Surface. 
We have already shown that two sets of rings may be seen 
by using a lens laid upon a slip of glass ; in which case, there- 
fore, whether we see the rings by reflection or by transmission, 
no more than four surfaces can be essential to their formation, 
in the following experiments for investigating the action of 
these surfaces I have preferred metalline reflection, when glass 
was not required, that the apparatus might be more simple. 
