344 Profs. C. V. Raman and Bhabonath Banerji on 



viewed through the completely dry film is remarkably similar 

 in appearance to the well-known diffusion-rings observed 

 around the focus of a thick concave mirror with a dusted 

 surface. The resemblance extends also to the case in which 

 the plate is held obliquely in front of the eye. As the plate 

 is gradually tilted, the halo runs out on one side, fresh 

 fringes appearing on that side of the source : and ultimately, 

 when the plate is held at a moderate obliquity, the halo 

 consists (at least in its brightest part) of a system of circular 

 arcs which are unequally spaced, the arc which passes through 

 the source being achromatic and those on either side of it 

 being strongly coloured in white light (see fig. 12 in Plate 

 Y.). [The oblique streak seen in the figure running across 

 the circular arcs was due to an accidental circumstance.] 

 In fact, the halo in its brightest part is very similar to the 

 diffusion rings due to a dusted mirror when the latter is 

 tilted. It should be mentioned, however, that in the outer 

 fainter parts of the halo (which do not appear in the photo- 

 graph reproduced) certain more complex effects are observed. 

 The detailed description of these may be deferred for the 

 present. 



When the film is only partially dry (as, for instance, in 

 fig. 8 in Plate V.), both sets of rings, that is those due to a 

 freshly prepared film and those shown by a completely dry 

 film, appear simultaneously, and owing to their superposition , 

 the phenomena appear somewhat confused, especially near 

 the centre of the halo. The effect of partial drying of the 

 film is also clearly noticeable in the diffraction-halo due 

 to an obliquely held plate. The elliptic or oval rings 

 appear traversed by a system of circular arcs running- 

 transverse to the plane of incidence. These are seen (some- 

 what faintly) in the photograph reproduced in fig. 10 in 

 Plate V. They become more and more prominent as the drying 

 of the film progresses, and ultimately remain alone in the field 

 (fig. 12, Plate V.) when the elliptic rings have disappeared. 



6. Non-Uniform Films. 



Standing in close relation with the phenomena described 

 above due to mixed plates of uniform thickness, are the effects 

 observed with mixed plates of variable thickness. The differ- 

 ence between the two cases is principally as regards the 

 method of observation. In the former case the film is placed 

 close to the eye, and the diffraction-halo surrounding a 

 distant source is observed. With mixed films of variable 

 thickness, on the other hand, the most suitable method of 



