830 Prof. C. Y. Raman and Mr. G. L. Datta on 



narrower as the incidence is increased till they reach a 

 minimum width, and then expand till they become very 

 wide again at grazing emergence. Stokes * in his paper 

 predicted from theory that this should occur, but, as already 

 mentioned, he was unable to observe any rings at such great 

 obliquities. It should be mentioned that the feature here 

 dealt with, namely the widening of the rings beyond a 

 certain point, depends only on the angle of emergence of the 

 scattered light, and not upon the angle of incidence of the 

 light from the source. Hence, even at normal incidence it is 

 possible to observe the effect. For this special case, it is 

 necessary to use a very powerful monochromatic light-source 

 (i. e. s a quartz-mercury vapour lamp with a green filter) and 

 to work in a dark room, so that the light scattered even 

 through 90° can be observed. 



A few measurements are here given of the width of 

 Quetelet's rings observed with mica to show the widening of 

 the rings towards the direction of grazing emergence, and 

 to exhibit the agreement of the observations with the 

 theory. 



Table I. 

 Quetelet's Rings in Mica. fi=l m 6 approximately. 



Angle of inci 

 on plate. 



dence 



Observed width of 



achromatic ring 



in arbitrary units. 



Calculated width. 



10° 





38 



39 



20° 





22 



21 



30° 





15 



14-8 



40° 





13 



12-5 



50° 





12 



12 



60° 





14-5 



131 



70° 





18 



17 



4. Quetelet's Rings by Multiple Reflexion, 



In observing the system of rings obtained with the air-film 

 held obliquely, a brilliant source of white light being used, 

 it was noticed that besides the usual system of which the 

 achromatic ring passes through the reflected image of the 

 source, three or four other systems of coloured bands con- 

 centric with it were also visible in the field, each with its 



* Loc. cit. Arts. 23-26 and 29. 



