130 iMISCHEL ON COLOURED RINGS. 



will make a central contact, either with the convexity or 



concavity of the subjacent glass, will always produce two sets 



of rings. 



4th. Lens on Fourth Method. A more refined but rather more difficult 



- on black -. - . . . ,. ,, . 



paper. wav °* seeiM £ tw ° se * s of rings is to lay a plain slip of ixlass 



on a piece of black paper, and when a convex lens is placed 

 upon the slip, there may be perceived, but not without parti- 

 cular attention, not only the first set, which has already been 

 pointed out as reflected from the first surface of the slip, but 

 also a faint secondary set from the lowest surface of the same 

 slip of glass. 



It will be less difficult to see two sets of rings by a reflec- 

 tion from both surfaces of the same glass, if we use, for in- 

 stance, a double concave of 8 inches focus with a double cont 

 vex of 7f inches placed upon it. For, as it is well known that 

 glass will reflect more light from the farthest surface when air 

 rather than a denser medium is in contact with it, the hollow 

 space o.f the 8-inch concave will give a pretty strong reflec- 

 tion of the secondary set. 

 •>th. Two pri- Fifth Method. The use that is intended to be made of two 

 pendent sets of sets °^ r ' n S s requires, that one of them should be dependent 

 rings* upon the other: this is a circumstance that will be explained 



hereafter, but the following instance, where two independent 

 sets of rings are given, will partly anticipate the subject. 

 When a double convex lens of 50 inches is laid down with a 

 slip of glass placed upon it, and another double convex one of 

 26 inches is then placed upon the slip, we get two sets of 

 rings of different sizes; the large rings are from the 50-inch 

 glass, the small lings from the 26- inch one. They arc to be 

 seen with great. ease, because they are each of them primary. 

 These may be By tilting the incumbent lens, or the slip of. glass, these two 

 TfcrTecT 3I sels °* r ' n fa s mav ^ e made to cross each other in any direc- 



tion,; the small set may belaid upon the large one, or either 

 of them may be separately removed towards any part of the 

 glass. This will be sufficient to show, that they have no con- 

 nection with each other. The phenomena of the motions, 

 and of the various colours and sizes assumed by these rings, 

 when different pressures and tiltings of the glasses are used, 

 will afford some entertainment. With the assistance of the 



shadow 



