S4 Experiments for investigating 



the fourtli shadow will come upon the second set, and in this 

 position of the apparatus it will become visible. 



Fourth Method. On a plain ujetalline 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 i'6-inch lens laid 

 upon the slip of glass will give thrqe ?ets of rings. Two of 

 them will easily be seen ; and when the shadow of the pen- 

 knife is held between them, the third set will also be per- 

 ceived. There is but one shadow visible in this arrange- 

 ment, which is the third ; the first and second shadows being 

 lost in the bright reflection from the mirror. 



Fifth Method. I placed a 6f-inch double convex upon an 

 8-inch double concave, and laid both together upon a plain 

 slip of glassw 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 centres. 



VI. Of four Sets of Rings. 



The difficulty of seeing many sets of rings increases with 

 their number, yet by a proper attention to the directions that 

 are given, four sets of concentric rings may be seen. 



First Method. Let a ^lip of glass, with a 26 -inch lens 

 laid upon it, be placed upon a piece of looking-glass. Under 

 one end of the slip, a small piece of wood one-tenth of an 

 inch thick must be put tokeep it from touching the looking- 

 glass. This arrangement will give us four sets of rings. The 

 first, third, and fourth may easily be seen, but the second set 

 will require some management. Of the three shadows which 

 this apparatus gives, the second and third must be brought 

 between the first and fourth sets of rings, in which situation 

 the second set of rings will become visible. 



Second Method. When three slips of glass are laid upon 

 a metalline mirror, and a plano-convex lens of about 17 

 inches focus is placed with its convex side upon them, four 

 sets of rings may be seen ; but this experiment requires a 

 very bright day, and very clean, highly polished slips of 

 plain glass. Nor caa it be successful unless all the fore- 

 going 



