ipo Dr. Herschei/s Experiments for investigating 
by a slight motion of the penknife, so as to make the third sha- 
dow of it go from one set to the other. 
Besides the use of the shadows, there is another way to make 
rings visible when they cannot be easily perceived, which is to 
take hold of the lens with both hands, to press it alternately a 
little more with one than with the other ; a tilting motion, 
given to the lens in this manner, will move the two sets of rings 
from side to side; and as it is well known that a faint object 
in motion may be sooner perceived than when it is at rest, 
both sets of rings will by these means be generally detected 
together. 
It will also contribute much to facilitate the method of seeing 
two sets of rings, if we receive the light in a more oblique 
angle of incidence, such as 40, 50, or even 60 degrees. This 
will increase the distance between the centers of the primary 
and secondary sets, and at the same time occasion a more 
copious reflection of light. 
Instead of a common looking-glass a convex glass mirror 
may be used, on which may be placed either a plain, a concave, 
or a convex surface of any lens or glass, and two sets of rings 
will be obtained. 
In the same manner, by laying upon a concave glass mirror 
a convex lens, we shall also have two sets of rings. 
The generalizations that have been mentioned when one 
set of rings was proposed to be obtained, may be easily applied 
with proper regulations, according to the circumstances of the 
case, not only to the method by glass mirrors already men- 
tioned, but likewise to all those that follow hereafter, and need 
not be particularized for the future. In the choice of the sur- 
faces to be joined, we have only to select such as will form a 
