pEUSCHEL ON COLOURED RINGS. 131 



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 pri- 

 mary one, while that from the 50-inch lens maybe 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 Three sets of 

 with some difficulty, hut by the assistance of the methods of rin g 3 - 

 tilting the glasses, and making use of the multiplied shadows 

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

 sufficient illumination of bright daylight. 



First Method. A 26 inch double convex lens placed upon lst A ] ens Qrl 

 three slips of plain glass will give three sets of rings. The three slips of 

 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 

 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; 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 2d. Alensand 



lens upon it, is placed upon a piece of good looking glass, a sl 'P of S ,ass 



n . , i /• ii-i oa lookhrg 



three sets of rings may be seen: the first and third sets are gi ass , 



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 sha- 

 dow 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 S(i Lens on 



• plain' metalline mirror, then a 26-inch lens placed upon the two glassy and 

 slips will produce three sets of rings ; but it is not very easy me u 

 to perceive them. By a tilting motion the third set will gene- 

 rally appear like a small white circle, which at a proper dis- 

 tance will follow the movement of the first set. As soon as 



I ihe first and third t-ets are in view, the third shadow of the peu- 



K 2 knife 



