860 Profs. C. V. Raman arid Bhabonatli Banerji on 



and the products fiyzSy, jiza:Ssc, and fjuxy&xSy are plotted 

 against the dry densities. It will be noted that the points 

 involving fjuxy lie sensibly on a straight line, whilst those 

 involving pyz and pzx lie approximately on two parallel 

 straight lines. 



Relations similar to the above have been obtained for each 

 of three values of Young's Modulus and six values of 

 Poisson's Ratio for spruce, which the writer hopes shortly 

 to publish, and these were found to agree with the sym- 

 metrical ratios of a body having three planes of elastic 

 symmetry *. It may be noted that the number of annual 

 rings per inch is not involved in any of the relations. 



The writer wishes to thank the late Principal, J. 0. M 

 Garnett, M.A., for providing facilities for conducting the 

 research, and also Professor Gr- Stoney, F.R.S., for many 

 helpful suggestions. He is further much indebted to Mr. A. 

 L. McAulay, B.Sc.Tas., for valuable assistance with the 

 experiments. 



LXXXI. On the Colours of Mixed Plates. — Part II. By 

 C. V. Raman, M.A., Palit Professor of Physics, and 

 Bhabonath Banerji, M.Sc, Assistant Professor of 

 Physics, University of Calcutta f . 



1. Inadequacy of the Elementary Diffraction Theory. 



THE optical phenomena exhibited by mixed plates have 

 been described in detail, with illustrations, in the first 

 part of the paper. We now proceed to consider their 

 explanation. It is obvious that the phenomena must be 

 classed as laminar diffraction effects. But, as has already 

 been remarked in the introduction to the first part of the 

 paper, many of the features observed differ from what one 

 might expect on the usual elementary theory of diffraction 

 phenomena. Conspicuous amongst these is the special 

 character of the diffraction-halo seen surrounding a distant 

 light-source viewed through a mixed plate of uniform thick- 

 ness. Many of the observed features of this halo are not 

 explicable on the elementary diffraction theory : namely, 

 the succession of dark and bright rings of widths rapidly 

 increasing from the centre outwards, the perfect blackness of 



* Love, ' The Mathematical Theory of Elasticity/ Art. 73. 

 t Communicated by the Authors. 



s 



