[ 838 ] 



XXX. On the Colours of Mixed Plates.— Part I. By C. V. 

 Raman, 21. A., Palit Professor of Physics, and Bhabonath 

 Banerji, 2I.Sc, Assistant Professor, University of Calcutta*. 



[Plates IV. & V.] 



1. Introduction. 



r jPHE colours exhibited by a mixed plate or film consisting 

 JL of two interspersed transparent media were investigated 

 by Thomas Young f, and were ascribed by him to the inter- 

 ference of the coherent streams of light passing through the 

 media and enrerging from the film having- suffered different 

 retardations. Later, it was pointed out by Brewster J that 

 the colours were due to laminar diffraction ; but his treat- 

 ment was not very complete, and apparently did not convince 

 later writers such as Yerdet and Mascart, who dealt with this 

 case in their treatises as one of simple interference, and 

 ignored the part played by diffraction. More recently 

 Profs. Charles Fabry § and R. W. Wood || have attempted 

 to give an explanation of the colours of mixed plates from the 

 standpoint of elementary diffraction theory. On investi- 

 gating the subject, however, we have found that there are 

 several features in the observed phenomena which the 

 treatments proposed fail to explain. For instance, when a 

 mixed plate consisting of a uniform film of liquid enclosing 

 a large number of air-bubbles of widely varying radii is 

 held in front of the eye, and a distant source of white 

 light is viewed through it, we should expect, according to 

 the theory given by Wood IT, that the halo seen surrounding 

 the source should be throughout of a more or less uniform 

 colour complementary to the tint of the regularly transmitted 

 light, and fluctuate as a whole when the thickness of the 

 film is varied. As will be seen in the following section, this 

 is very far indeed from being in agreement with what is 

 actually observed. Further, the experimental examination 

 of the subject which we have carried out has brought to 

 light a number of interesting features which appear hitherto 

 to have been overlooked. It is proposed, in Part I. of this 

 paper, to give a general description of the observed effects. 



* Communicated by the Authors. 



f Phil. Trans. Boy. Soc. 1802, p. 390, and < Elements of Natural Philo- 

 sophy,' vols. i. & ii. 



t Phil. Trans. Roy. Soc. lS38,p. 73. 



§ Journal cle Physique, viii. p. 595 (1899). 



|| Phil. Mag. April 1904, and ' Physical Optics,' 2nd Edition, p. 252. 



*f[ Zoc-. cit. 



