370 Prof. J. C. McLennan on Diffusion of Actinium 



slope, but while the line for deep red does not change, that 

 for blue becomes horizontal, or critical frequency becomes 

 independent of illumination. The Purkinje effect and its 

 opposite follow at once from these facts. 



5. The flicker photometer is shown to be largely influenced 

 by the critical frequency phenomena, but not to obey the 

 simple law which would follow were it a mere dove-tailing 

 of two pure flickers. 



6. The peripheral retina is found to be more sensitive to 

 flicker only for momentary observation before adaptation or 

 fatigue sets in. The fovea is more sensitive to red flicker, 

 the periphery to blue, and the difference is more striking at 

 low illuminations, as noticed by Dow. 



7. The phenomena of critical frequency are in general 

 accord with Koenig's theory of the function of the visual 

 purple and with the hypothesis that the retinal cones are 

 chiefly active in the case of intermittent or alternating 

 stimuli. 



Physical Laboratory, 



National Electric Lamp Association, 



Cleveland, Ohio. 



January, 1912. 



XXXIL On the Diffusion of Actinium Emanation and the 

 Active Deposit produced by it. By Prof. J. C. McLennan*. 



IN a paper by W, T. Kennedy, published in the Phil. Mag. 

 of November 1909, an account is given of some experi- 

 ments on the active deposit products of actinium. 



In one series of experiments observations were made on 

 the distribution of the active deposit obtained on two parallel 

 plates, 2 mm. apart and charged to 250 volts, the actinium 

 salt being so placed immediately below the plates that the 

 emanation from it was compelled to diffuse up between them. 

 In these experiments the plates and the salt were inserted in 

 a vessel in which the air could be maintained at any selected 

 pressure, and the observations were made on the distribution 

 of the active deposit obtained on the plates with air-pressures 

 of 760 mm., 120 mm., 25 mm., and 5 mm. respectively. The 

 distributions of the deposit obtained at atmospheric pressure 

 on the negatively charged plate, and also on the positively 

 charged one, are shown in fig. 1. From the curves it will 

 be seen that on both the positively charged plate and on the 



* Communicated by the Author. Presented to the Royal Society of 

 Canada, May 17, 1912. 



