the Cause of coloured concentric Rings, 83 



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. 



KV •'/; 'ii> ■:):;^f(fj •■\\ 



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 sha- 

 dows of a penknife, we may see them very well, when there 

 is a sufficient illumination of bright daylight. '' 



^^Wtrst 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, 

 otherwise 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 circumstances are favourable, the third, which is the 

 faintest, will also appear. If it cannot be seen, some of the 

 compound shadows of the penknife must be thrown upon 

 it 5 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 34- 

 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 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 ge- 

 nerally appear like a small white circle, which at a proper 

 distsfnce will follow the movement of the first set. Ais 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 



F 2 the 



