the Cause of coloured concentric Rings. 193 
when different pressures and tiltings of the glasses are used 
will afford some entertainment. With the assistance of the 
shadow of the penknife the secondary set belonging to the 
rings from the 26-inch lens will be added to the other two sets; 
but in tilting the glasses this set will never leave its primary 
one, while that from the 50-inch lens may be made to go any 
where across the other two. 
V. Of three Sets of Rings. 
To see three sets of concentric rings at once is attended with 
some difficulty, but by the assistance of the methods of tilting 
the glasses and making use of the multiplied shadows of a 
penknife we may see them very well, when there is a suffici- 
ent illumination of bright daylight. 
First Method. A 26-inch double convex lens placed upon 
three slips of plain glass will give three sets of rings. The 
slips of glass should be nearly 2-tenths of an inch thick, other- 
wise the different sets will not be sufficiently separated. When 
all the glasses are in full contact the first and second sets may 
be seen with a little pressure and a small motion, and, if cir- 
cumstances are favourable the third, which is the faintest, will 
also appear. If it cannot be seen, some of the compound sha- 
dows of the penknife must be thrown upon it ; for in this case 
there will be five shadows visible, several of which will fall 
together and give different intensity to their mixture. 
Second Method. When a single slip of glass, with a g^-inch 
lens upon it, is placed upon a piece of good looking-glass, three 
sets of rings may be seen : the first and third sets are pretty 
bright, and will be perceived by only pressing the lens a little 
upon the slip of glass ; after which it will be easy to find the 
