680 Prof. 0. V. Raman and Mr. K. Seshagiri Rao on 



discussion o£ these and other effects that the theory of the 

 colours of mixed plates usually put forward is inadequate, 

 and a new treatment was suggested in order to explain 

 the observed phenomena. We jiow proceed to summarize 

 the theory, to develop its consequences in detail, and to 

 show that the results obtained are in quantitative agreement 

 with experiment. 



2. Statement of Theory 'proposed. 



The explanation of the various phenomena exhibited by 

 mixed plates is closely related to that of the effects observed 

 in one fundamental case — that is, the character of the 

 diffraction halo seen surrounding a distant source of light 

 when viewed through a mixed plate of uniform thickness 

 held normally close to the eye. The remarkable feature 

 is that though the bubbles of air enclosed in the film 

 forming the mixed plate vary, arbitrarily in size and shape 

 and are irregularly arranged in the film, nevertheless the 

 diffraction halo seen in this case exhibits a regular structure 

 consistino- of a series of circular rings which are closer 



e> # "... 



together in the centre of the halo and wider apart in its 

 margin, the number of such rings depending only on the 

 thickness of the film and its composition. The theory 

 proposed is that these effects are connected with the special 

 character of the laminar edges in the film. Owing to the 

 action of surface tension, the edges of the bubbles are not 

 cylindrical, but are drawn inwards as a meniscus, the form 

 of which depends on the angle of contact between the 

 liquid and plate. If the angle of contact be assumed to 

 be zero, the cross-section of the meniscus should be semi- 

 circular. The investigation on this basis (given in the 

 second paper of the series) indicates that the intensity of 

 the light scattered in its passage through the film at these 

 laminar edges should exhibit maxima and minima in various 

 directions besides being strongly asymmetrical in its dis- 

 tribution — that is, differing very greatly in directions lying 

 on either side of the regularly transmitted light. For very 

 small angles of diffraction — that is, in directions very nearly 

 coincident with that of regular propagation — the results of 

 the investigation agree with the usual elementary theory ; 

 but in other directions differ entirely from it. Considering 

 the aggregate effect of all the elementary laminar boundaries 

 assumed to be irregularly arranged .in the film, it is shown 

 that the scattered light would give rise to two sets of 

 bright and dark circular rings in the halo, the first set 

 extending np to very large angles of diffraction, and tl e 



