244 THE ROYAL SOCIETY OF CANADA 



A photograph of the fluorescing crystals was obtained by placing 

 them on the sensitive side of a plate and subjecting them to the 

 influence of gamma rays for one hour. The fluorescence is shown in 

 the bright lines around the edges of the crystals. After removing 

 the source of exciting rays, the crystals were placed on another 

 photographic plate, but the only impression then made was that of 

 diamond No. 1, the only crystal which was phosphorescent. 



(d) Photo-electric Conductivity. 



Upon preliminary examination with heterochromatic light, it 

 was found that of the six diamonds, only samples No. 1 and No. 2 

 showed a change in conductivity when exposed to illumination. 

 With the other diamonds, no appreciable currents were registered 

 in the most intense available light and in fields as high as 11,500 

 volts per cm. 



4. Photo-electric Properties of Diamonds. 



(a) Conductivity of Unilluminated Diamond No. 1. 



On applying a field to the unilluminated crystal, a small current 

 was observed. When the direction of the field was reversed, this 

 current was absent, showing that the crystal was conducting in one 

 direction. On illumination, a photo-electric current resulted irres- 

 pective of the direction of the external field, but of much greater 

 magnitude in the direction in which the "dark" current flowed. 

 This property of unidirectional conductivity has been observed in 

 other light sensitive substances, and its connection with the photo- 

 electric phenomenon will be discussed later. 



{h) Effect of Time of Exposure on Photo-electric Current. 



On illuminating the crystal, it was found that the photo-electric 

 current (measured by subtracting from the total galvanometer 

 deflection that due to the dark current) increased with time, rising 

 slowly to a maximum. The lag of photo-electric response varied 

 considerably with wave-length of exciting light, and the time for 

 complete recovery was always longer than that of exposure. 



Time-current curves shown in Fig. 2 are typical. In taking 

 observations the light was shut off when the galvanometer deflection 

 reached its first maximum. Equilibrium conditions could not be 

 obtained, for with passage of time came irregularities in the motion 

 of the image on the scale which reached such magnitudes as to make 

 readings impossible. The broken curve in Fig. 2 is typical of the 

 behaviour of the current after a long time of exposure to light. 



