ig2 Dr. Herschel’s Experiments for investigating 
particular attention, not only the first set, which has already 
been pointed out as reflected from the first surface of the slip, 
but also a faint secondary set from the lowest surface of the 
same slip of glass. 
It will be less difficult to see two sets of rings by a reflec- 
tion from both surfaces of the same glass, if we use, for in- 
stance, a double concave of 8 inches focus with a double con- 
vex of 7|- inches placed upon it. For, as it is well known 
that glass will reflect more light from the farthest surface 
when air rather than a denser medium is in contact with it, 
the hollow space of the 8-inch concave will give a pretty 
strong reflection of the secondary set. 
Fifth Method. The use that is intended to be made of two 
sets of rings requires that one of them should be dependent 
upon the other : this is a circumstance that will be explained 
hereafter, but the following instance, where two independent 
sets of rings are given, will partly anticipate the subject. 
When a double convex lens of 50 inches is laid down with a 
slip of glass placed upon it, and another double convex one of 
26 inches is then placed upon the slip, we get two sets of 
rings of different sizes ; the large rings are from the 50-inch 
glass, the small rings from the ab-inch one. They are to be 
seen with great ease, because they are each of them primary. 
By tilting the incumbent lens or the slip of glass these two 
sets of rings may be made to cross each other in any direc- 
tion ; the small set may be laid upon the large one, or either 
of them may be separately removed towards any part of the 
glass. This will be sufficient to show that they have no con- 
nection with each other. The phenomena of the motions, 
and of the various colours and sizes assumed by these rings. 
