the Cause of coloured concentric Rings . 187 
by reflection will also give them by transmission, when ex- 
posed to the light in any of the three ways that have now been 
pointed out. When these are added to the former, it will be 
allowed that we have an extensive variety of arrangements 
for every desirable purpose of making experiments upon rings, 
as far as single sets of them are concerned. 
III. Of Shadows. 
When two or more sets of rings are to be seen, it will re- 
quire some artificial means, not only to examine them criticaHy, 
but even to perceive them ; and here the shadow of some 
slender opaque body will be of eminent service. To cast 
shadows of a proper size and upon places where they are 
wanted, a pointed penknife may be used as follows. 
When a plain slip of glass or convex lens is laid down, and 
the point of a penknife is brought over either of them, it will 
cast two shadows, one of which may be seen on the first sur- 
face of the glass or lens, and the other on the lowest. 
When two slips of glass are laid upon each other, or a con- 
vex lens upon one slip, so that both are in contact, the pen- 
knife will give three shadows; but if the convex lens should 
be of a very short focus, or the slips of glass a little separated, 
four of them may be perceived ; for in that case there will be one 
formed on the lowest surface of the incumbent glass or lens ; 
but in my distinction of shadows this will not be noticed. Of 
the three shadows thus formed the second will be darker than 
the first, but the third will be faint. When a piece of looking 
glass is substituted for the lowest slip the third shadow will be 
the strongest. 
Three slips of glass in contact, or two slips With a lens 
