T9-1 Dr. Herschel’s Experiments for investigating 
second set with the assistance of the proper shadow. In this 
case four shadows will be seen ; and when the third shadow 
is upon the first set, the fourth will be over the second set and 
render it visible. 
Third Method. When two slips of glass are laid upon a 
plain metalline mirror, then a 26-inch lens placed upon the 
slips will produce three sets of rings ; but it is not very easy to 
perceive them. By a tilting motion the third set will generally 
appear like a small white circle, which at a proper distance 
will follow the movement of the first set. As soon as the first 
and third sets are in view the third shadow of the penknife 
may be brought over the first set, by which means the fourth 
shadow^ will come upon the second set, and in this position of 
the apparatus it will become visible. 
Fourth Method. On a plain metalline mirror lay one slip of 
glass, but with a small piece of wood at one end under it, so that 
it may be kept about one-tenth of an inch from the mirror, and 
form an inclined plane. A 26-inch lens laid upon the slip of 
glass will give three sets of rings. Two of them will easily be 
seen ; and when the shadow of the penknife is held between 
them the third set will also be perceived. There is but one sha- 
dow visible in this arrangement, which is the third ; the first and 
second shadows being lost in the bright reflection from the 
mirror. 
Fifth Method. I placed a 6|-inch double convex upon an 
8-inch double concave, and laid both together upon a plain slip 
of glass. This arrangement gave three sets of rings. They 
may be seen without the assistance of shadows, by using only 
pressure and tilting. The first had a black and the other two 
had white centers. 
