On Quetelet's Rings and otlier Allied Phenomena. 527 



2. Observations of Qaetelefs Rings at Oblique Incidences. 



So far as the authors are aware, no observations have been 

 published of Quetelet's rings with plates on which light is 

 incident at large obliquities. Stokes in his paper * describes 

 au unsuccessful attempt in this direction — "As the angle of 

 incidence increases, the bands become finer and finer, and 

 after they have become too fine to be distinguished by the 

 naked eye, they may still be seen through a small telescope, 

 provided the source of light be sufficiently small. ... I saw 

 traces of the bands when the angle of incidence was about 

 25° 40', but they were not at all well-formed beyond an 

 angle of about 10° 40', after which they began to be con- 

 founded with rays which shot in all directions from the 

 image of the luminous point." He also suggests that with 

 a thinner mirror the observations might probably have been 

 pushed further. 



Fig. 1. 



As remarked by Stokes, the difficulty in observing the 

 rings given by a plate at any considerable obliquity arises 

 from the structure of the luminous field in which they should 



* Loc. cit. p. 184. 

 3 I 2 



