834 Prof. C. V. Raman and Mr. G. L. Datta on 



ellipses in shape. The curves of blurring or minimum 

 visibility along which the maxima and minima respectively 

 of the two sets are superposed, have the same form as the 

 '■ isochromatic lines" in polarized light of a plate of twice 

 the thickness *. From the photographs of the Haidinger's 

 rings in mica secured by Chinmayanandam, it is seen that 

 along these curves of minimum visibility the visible ring- 

 system formed by the superposition of two sets of rings 

 appears dislocated — that is, the minima on the side of a curve 

 of minimum visibility run into the maxima on the other side, 

 and vice versa. 



Examination of the Quetelet's rings in white light exhibited 

 by a sheet of mica having one surface dimmed by a film of 

 ammonium chloride, showed somewhat similar phenomena. 

 For this purpose it is desirable to use a fairly bright source 

 of light, and work in a darkened room so that the complete 

 rings may be observed. When the incidence on the mica is 

 normal, nothing special is noticed : but when the mica is 

 tilted somewhat, the ring-system expands, and it will be 

 noticed at once that the coloured circles appear dislocated 

 along certain curved arcs, the bright rings on one side 

 running into the dark rings on the other, and vice versa. 

 When the plane containing the optic axes of the mica is 

 parallel or perpendicular to the plane of incidence, the dis- 

 locations occur in a symmetrical manner with reference to 

 the diameter of the rings passing through the reflected image 

 of the source ; but in other cases the dislocations occur in 

 regions unsymmetrically situated with reference to this 

 Hne. 



It should be understood that the form and position of the 

 lines of minimum visibility in Quetelef s rings are by no 

 means necessarily the same as for Haidinger's rings in the 

 same plate. Though the spacing of the rings is practically 

 identical in the two cases, the exact positions of the rings 

 (for either component of polarization) may differ slightly, the 

 formula giving the absolute positions of the rings being 

 different (see equations (1) and (2) above). This may result 

 in a very considerable shift of the curve along which the 

 maxima and minima respectively of the two sets of rings 

 coincide. The fact that one of the rays on emergence is 

 scattered may also introduce complications in regard to its 

 polarization that do not arise in the theory of Haidinger's 



* Chimnajanandam, loc, cit. 



