the Cause of coloured concentric Rings . 395 
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 slip of glass, with a 20-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 to keep 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 
can it be successful unless all the foregoing methods of seeing 
multiplied sets of rings are become familiar and easy. 
I have seen occasionally, not only four and five, but even 
six sets of concentric rings, from a very simple arrangement of 
glasses : they arise from reiterated internal reflections ; but it 
will not be necessary to carry this account of seeing multiplied 
sets of rings to a greater length. 
VII. Of the Size of the Rings. 
The diameter of the concentric rings depends upon the 
