Lord Rayleigh : Coloured Photometry. 301 



is seen that the theoretical curve for mercury vapour agrees 

 fairly well with the experimental one. It seems to vary, 

 however, more from the experimental curve than any of the 

 curves for the other gases and vapours for which it has 

 been tried. 



By measuring the areas under the experimental ionization 

 curves, I. and II., and finding the ratio of the area under the 

 air curve to the area under the mercury vapour curve, the 

 ratio of the amount of energy required to produce an ion in 

 mercury vapour to that required to produce an ion in air 

 was found to be 072. That is, it requires only 0*72 as 

 much energy to produce an ion in mercury vapour as it 

 does in air. 



In conclusion I wish to express my thanks to Professor A. 

 P. Carman for furnishing me the facilities by means of which 

 it was made possible to carry out this research. 



Results. 



1. The ionization curve for the alpha particles from 

 polonium has been determined in mercury vapour, and found 

 to be of the general type as has been found for other gases 

 and vapours*. 



2. The alpha particle loses 0*72 as much energy in pro- 

 ducing an ion in mercury vapour as it does in air. 



Physics Buildings, 

 Urbana, Illinois, 



June 6, 1912. 



XXIV. Coloured Photometry. 

 By Lord Payleigh, O.M., F.K.S.t 



IN his recent paper on the Photometry of Lights of: 

 Different Colours J Mr. H.Ives remarks: — " Xo satis- 

 factory theory of the action of the flicker photometer can 

 be said to exist. What does it actually measure ? We may 

 assume the existence of a ' luminosity sense ' distinct from 

 the colour sense. ..... If , for instance, there exists a physio- 

 logical process called into action both by coloured and 

 uncoloured light, a measure of this would be a measure of 

 a common property." 



Very many years ago it occurred to me that the adjustment 



* Loc cit. 



t Communicated by the Author. 



| Phil. Mag. vol. xxiv. p. 178. 



