426 Lord Rayleigh on the Colours diffusely 



No more than Rowland have I succeeded in getting dif- 

 fusely reflected colours from collodion films on glass or, I may 

 add, quartz, either with or without the treatment with the 

 breath suggested by Wood. The latter observer describes 

 an experiment (p. 174) in which a film, deposited on the 

 face of a prism, frilled under the action of the breath and 

 then afforded a nearly three-fold reflexion. But, as I under- 

 stand it, this augmented reflexion was specular. The only 

 thing that I have seen at all resembling this was when I 

 treated a coated glass with dilute hydrofluoric acid with the 

 intention of loosening the film. Even when dry, the film 

 remained out of optical contact with the glass, except I 

 suppose at detached points, and gave an augmented specular 

 reflexion, as was to be expected, inasmuch as three surfaces 

 were operative. 



Two views are possible with regard to the different 

 behaviour of films on metal and on glass. One is to sup- 

 pose that the actual structure is different in the two cases ; 

 the other, apparently favoured by Wood, refers the differ- 

 ence to the copious reflexion of light from metallic surfaces. 

 The first view would seem the more probable a priori and is 

 to a certain extent supported by Rowland's experiment. 

 I have not succeeded in carrying out any decisive test. 

 On either view we may expect the result to be modified by 

 the metallic reflexion. 



As to the explanation of the colours, anything more than 

 a rough outline can hardly be expected. We do not know 

 with any precision the constitution of the film as modified 

 by frilling. And, even if we did, a rigorous calculation of 

 the consequences would probably be impracticable. But 

 some idea may be gained from considering the action of an 

 obstacle, e. g. a sphere, of material slightly differing optically 

 from its environment and situated in the neighbourhood of a 

 perfectly reflecting plane surface upon which the light is 

 incident perpendicularly. Under this condition the reflected 

 light may still be supposed to consist of plane waves un- 

 disturbed by the previous passage through and past the 

 obstacle. 



The calculation, applying in the absence of a reflector but 

 without limitation to the spherical form of obstacle, was 

 given in an early paper *. In Maxwell's notation, /, g, h are 

 the electric displacements. The magnetic susceptibility is 

 supposed to be uniform throughout; the specific inductive 

 capacity to be K, altered within the obstacle to K-f-AK. 



* " On the Electro-magnetic Theory of Light," Phil. Mag. vol. xii. 

 p. 81 (1881) ; Scientific Papers, vol. i. p. 518. 



