^%, Experiments for investigating 



Fourth Method. A more refined but rather more difficult 

 way of seeing two sets of rings, is to lay a plain slip of glass 

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

 upon the slip, there maybe perceived, but not without par- 

 ticular attention, not only th'e 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- 

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

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

 convex of /j inches placed upon it. For, as it is well known 

 that glass will reflect more light from the furthest surface 

 when air rather than a denser medium is in contact with it, 

 the hollow space of the 8- inch concave will give a pretty 

 strong reflection of the secondary set. 



Fifth Method. The use that is intended to be made of two 

 sets of rings requires that one of them should be dependent 

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

 hereafter; but the following instance, where two indepen- 

 dent 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 s^lass 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 rings from the 26-inch one. They are to be 

 seen with great ease, because they are each of them primar}\ 

 By tilting the incumbent lens or the slip of glass these two 

 sets of rino^s may be made to cross each other in any direc- 

 tion ; the small set may be laid 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 phaenomena of the motions, 

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

 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; 



