Queteletfs Rings and other Allied Phenomena. 831 



own achromatic band and the usual colour-sequence, but 

 of different widths. The appearance in the plane of incidence 

 is diagramatically illustrated in fig 2. 



The position of the reflected image and the usual system 

 of coloured bands on either side of it is indicated by (1). 

 The symbols (%) and (J) indicate two other systems, the 

 fringe-widths of which are smaller roughly in the ratio of 

 the fractions which indicate them. (2) and (3) are two 

 more systems, of which the latter is, in general, somewhat 

 less easy to observe. These are of greater fringe-width 

 than(l). 



Fig. 2. 



ttertno 

 /isyer. 



surface 



The explanation of the formation and character of these 

 new systems of Quetelet's rings is interesting. In the usual 

 (primary) system of Quetelet's rings, each of the interfering 

 pencils suffers a single reflexion at the silvered surface and 

 passes through the plate twice. The systems now described 

 arise in a different way, one of the interfering pencils passing 

 twice through the plate and suffering a single reflexion, and 

 the other passing four or six times through the plate. 

 suffering multiple reflexion. To illustrate this, the paths 



