. Manchester Memoirs, Vol, Ixiii. (1919) No. 4 19 



resultant pattern R for photo-reproduction purposes, and the corre- 

 sponding value of ^, are not investigated. Such work would seem to 

 be largely a matter for experiment as well as theory, and is engaging 

 the attention of Mr. Fishenden and the author. 



Finally, the author would like to give his thanks to Mr. Fishenden 

 for the assistance he has given in preparing plates, etc., and thus 

 enabling the theory to be tested ; also for many suggestions in 

 connection with the paper. 



The author would also thank Mr. L. J. Mordell for valuable 

 criticisms concerning the Appendix. 



