Long. Wave Limits of Normal Photoelectric Effect. 47& 

 formula for o> of the form 



be 2 /m 



2 ( 2 i£VZ!LY( 2 -£>L) 



and it appears to satisfy the two tests so well that there can 

 hardly be any doubt as to its correctness. I have shown in 

 my previous communications, how well it explains most of 

 the variations observed in the specific rotation of a solution 

 consequent on a variation of the constitution of that solution , 

 and although one or two small difficulties were then met 

 with, they may be overcome by the introduction of the effects 

 due to more or less absorption of the light. This formula 

 satisfies the second test equally well. In fact LommelV 

 formula is precisely the formula furnished by the present 

 theory as a first approximation in the particular case where 

 there is only one absorption band in the whole spectrum of 

 the substance at an appreciable distance from the position in 

 the spectrum corresponding to the frequency n. 



It would, therefore, appear that the test of reality results 

 entirely in favour of the third theory, and the formula 

 deduced from it may therefore be taken as best representing 

 the facts. 



It may, of course, be argued that all three theories are the 

 same in the end ; it is only a matter of: constants ! However,, 

 when, as in the present case, the chief object is to investigate 

 the nature of these constants, it is very essential that we 

 should have a clear view of the form in which they are first 

 introduced into the theory, the ultimate object being to 

 obtain an explanation of their existence at all, in the form 

 finally adopted for them. 



Sheffield, Nov. 1913. 



LIV. On the Long- Wave Limits of the Normal 

 Photoelectric Effect, 



To the Editors of the Philosophical Magazine. 



Gentlemen, 



IN the December issue of the Philosophical Magazine,. 

 Drs. Pohl and Pringsheim discussed the present position 

 of certain questions in photo-electricity, and concluded 

 that there was not sufficient evidence to determine the 

 relation between the maximum emission velocity of photo- 

 electrons and the frequency of the light. It has long been 



