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



fluctuations, however, occur only when the dark ring is near 

 the centre, and become imperceptible when it has moved to 

 the position of the third or fourth • ring from the centre, 

 beyond which the dark rings are all more or less perfectly 

 black. At the same time, the form of the rings undergoes 

 alteration, becoming elliptical near the centre and o f an 

 oval form in the outer parts of the halo. With increasing 

 obliquity these ovals assume unsymmetrical shapes, the 

 curvatures at the two ends of the minor axis (which lies in 

 the plane of incidence) being unequal. This effect is shown 

 in fig. 7 in Plate V. Indeed, on the flatter side the ovals 

 may actually straighten out and even reverse their curvature — 

 that is, become concave outwards; while on the other side, 

 which corresponds to directions more nearly parallel to the 

 plate, the rings remain convex outwards. At the same time, 

 the different parts of the rings in the halo appear very unequal 

 in their illumination, the light becoming more or less com- 

 pletely concentrated in the plane of incidence and the flatter 

 side of the rings appearing brighter than the more strongly 

 curved side (fig. 7 in Plate V.). 



4. Films containing Distorted Boundaries. 



Mixed plates freshly prepared by rubbing up saliva or white 

 of egg between glass plates generally show, when examined 

 under the microscope, that the air-bubbles tend to take up 

 a circular form, or at any rate do not show a bias towards 

 elongation in any particular direction. But if the glass 

 plates enclosing a mixed film of white of egg and air are 

 pressed together and continuously moved over each other in 

 any one direction, the bubbles in the film become distorted, 

 assuming elliptic or oval shapes of which the major axis is in 

 a direction perpendicular to that of movement of the plates. 

 Fig. 6 in Plate IV. is a micro-photograph of a film obtained 

 in this manner. When the diffraction-halo due to such a 

 plate (normally held) is observed, it is found that the rings 

 in the halo retain their relative position and circular shape, 

 the only conspicuous effect of the altered form of the 

 boundaries being to increase the luminosity of the halo in. 

 directions transverse to the preponderating direction of- the 

 boundaries. This is illustrated in fig. 5. This observation, 

 taken along with those described in the preceding section, 

 clearly shows that the essential feature of the halo, namely, 

 the succession of dark and bright rings of widths increasing 

 from the centre outwards, does not depend for its formation 

 on the position, size, or shape of the air-bubbles in the film, 



