the Colours of Mixed Plates. 861 



the dark rings in the outer part of the halo, and the obviously 

 composite structure of the inner part of the halo. Jn 

 seeking for an explanation of these effects, a clue is furnished 

 by the observation already recorded, that when a mixed 

 plate is observed by the light diffracted by it, the whole 

 surface of the film does not appear luminous, but only the 

 laminar boundaries or lines of separation of the two media 

 forming the film. The optical effects of mixed plates are thus, 

 in fact, the optical effects due to the scattering or radiation 

 of light from laminar diffracting boundaries. It is necessary 

 to study the manner in which an individual laminar boun- 

 dary scatters or diffracts light incident on it ; to compare 

 this with the indications of theory, and from the observed 

 effects to infer the aggregate result of the scattering by a 

 large number of such boundaries irregularly situated on the 

 film. These points we now proceed to discuss. 



2. Examination of Mixed Plates by the Method of the 

 ■ Foucault Test. 



The most convenient way of examining the scattering of 

 light by a laminar boundary in directions nearly coincident 

 with that of the incident waves is by the method of observation 

 known as the Foucault knife-edge test or the Topler Schlieren 

 method. The theory of this method was developed by the 

 late Lord Eayleigh on the basis of the usual elementary 

 treatment of diffraction phenomena *, and it was shown by 

 him that a discontinuous laminar boundary should appear as 

 a luminous line when examined by the method of the Foucault 

 test. A beautiful illustration of Rayleigh's theory is fur- 

 nished on examining a clear piece of mica by the Foucault 

 test, when it will be found that the strise or boundaries in 

 the mica between regions having slightly different thick- 

 nesses shine out as vividly-coloured lines cf light in a dark 

 field f. To study the phenomena of mixed plates by a 

 similar method, the following arrangement is suitable. 

 Light from a small circular aperture illuminated by an 

 incandescent filament lamp falls upon a good achromatic 

 lens, and is brought to a focus at a distance from it. Two 



* '•' On Methods for Detecting- Small Optical Retardations and the 

 Theory of the Foucault Test." Phil. Mag-. Feb. 1917. 



t "On the Colours of the Strire in Mica." 0. V. Raman and P. N. 

 Ghosh. 'Nature,' October 1918 See also' P. N. Ghosh, Woe. Roy. 

 Soc. A. Series, vol. xcvi. p. 257 (1919). 



Phil. Mag. S. 6. Vol. 41. No. 246. June 1021 3 L 



