340 Profs. C. Y. Raman and Bhabonath Banerji on 



fig. 1 in Plate IV.) The laminar boundaries appear as black 

 lines in transmitted light under the microscope. 



Using the method of observation described above and a 

 freshly prepared plate, some remarkable effects are observed 

 which will now be described. 



Observations in Monochromatic Light. 



Surrounding the source is seen a brilliant halo, which, if 

 the source is sufficiently small and distant, shows especially 

 near the centre a finely mottled or granular structure. The 

 halo consists of a succession of dark and bright circular rings 

 of which the number and position depend on the thickness of 

 the film. With thick films fifteen or twenty rings may 

 easily be obtained, with thin films a proportionately smaller 

 number. The most remarkable features of the ring-system 

 are : firstly, that the successive rings are closer together near 

 the centre of the halo and wider apart towards the margin of 

 the halo, where they are very broad and faint ; secondly, the 

 dark rings in the halo are all more or less perfectly black, 

 except the two or three rings nearest the centre of the halo which 

 are not so dark as the rest, and fluctuate in sharpness and 

 intensity with their exact position in the halo. Figs. 2 and 3 

 in Plate IV. show the ring-system above described for a 

 moderately thin film. The photographs of the halo here 

 reproduced were secured by using a short-focus wide-angle 

 lens (Zeiss-Tessar F/3"4lens of focal length 5 cm.), the mixed 

 plate being placed as close to the lens as possible. Fig. 2 is 

 a light print showing the faint and broad outer rings, and 

 fig. 3 a deep print showing more clearly the part of the halo 

 near the centre. 



When the mixed plate is moved in its own plane so as to 

 bring a thinner part of the film in front of the eye, the rings 

 seen in the diffraction halo move inwards, closing up at the 

 centre so that the number visible in the field decreases and 

 the rings appear wider apart. As each of the dark rings in 

 the inner part of the halo contracts and moves inwards, it 

 undergoes a periodic fluctuation of intensity, becoming 

 alternately broad and diffuse, and then sharp and black, 

 just as it is about to close up at the centre. Furtlfer away 

 from the centre, however, the rings merely contract and 

 move inwards without any noticeable change in their ap- 

 pearance. In fact, what is seen distinctly suggests that 

 near the centre of the halo there is a second and fainter ring- 

 system superposed on the first, with the result that in this 

 region the dark rings vary in sharpness and intensity 

 according as, at a given point, the two sets of rings are in or 



