*88 Dr. Herschel’s Experiments for investigating 
upon them, or also a looking glass, a slip and a lens put together, 
will give four shadows, one from each upper surface and one 
from the bottom of the lowest of them. 
In all these cases a metalline mirror may be laid under the 
same arrangement without adding to the number of sha- 
dows, its effect being only to render them more intense and 
distinct. 
The shadows may be distinguished by the following method. 
When the point of the penknife is made to touch the surface 
of the uppermost glass or lens, it will touch the point of its 
own shadow, which may thus at any time be easily ascertained : 
and this in all cases I call the first shadow ; that which is next 
to it, the second ; after which follows the third, and so on. 
In receding from the point, the shadows will mix together, 
and thus become more intense ; but which, or how many of 
them are united together, may always be known by the points 
of the shadows. 
When a shadow is to be thrown upon any required place, 
hold the penknife nearly half an inch above the glasses, and 
advance its edge foremost gradually towards the incident light. 
The front should be held a little downwards to keep the light 
^rom the underside of the penknife, and the shadows to be 
used should be obtained from a narrow part of it. 
With this preparatory information it will be easy to point 
out the use that is to be made of the shadows when they are 
wanted. 
IV. Of two sets of Rings. 
I shall now proceed to describe a somewhat more complica- 
ted way of observation, by which two complete sets of concen- 
tric rings may be seen at once. The new $r additional set 
