Photometry of Lights of Different Colours. 149 



When we consider that the constituents of any one such 

 beam would have a wide range of hardness and that the 

 hardest constituents are more effective in transforming the 

 incident radiation, it is not to be wondered that on the whole 

 the scattered radiation behaved as a primary weakened in 

 intensity only. 



The authors recognize that their experimental data are 

 somewhat fragmentary and fail to provide a complete 

 solution of this peculiarly elusive problem. It was thought, 

 however, that the results already obtained contained sufficient 

 novelty to warrant publication. 



Summa?*y. 



It is shown that : (1) A homogeneous beam when scattered 

 by a substance of low atomic weight is transformed into a 

 softer type of radiation. 



(2) The harder the exciting beam, the greater is the 

 intensity of the scattered radiation. 



(3) The harder the exciting beam, the more profound is 

 the change in its quality. 



In conclusion we wish to thank Professor Wilberforce for 

 placing the resources of the laboratories at our disposal. 



The George -Holt Physical Laboratories. 

 January 1912. 



XII. Studies in the Photometry of Lights of Different Colours. 

 By Herbert E. Ives*. 



[Plates III.-V.] 

 I. 



Spectral Luminosity Curves obtained by the Equality of 

 Brightness Photometer and the Flicker Photometer under 

 similar conditions. 



Synopsis. 

 Introduction. 



1. Difficulties in determination of luminosity in the presence of colour 



difference. 



2. Existing methods of heterochromatic photometry. 



a. Equality of brightness. 



b. Visual acuity. 



c. Critical frequency. 



d. Flicker photometer. 



* Communicated h\ the Author. 



