iLEViJ PHOTO-ELECTRIC CONDUCTIVITY OF DIAMOND 253 



That there is a connection between phosphorescence and photo- 

 sensitivity is suggested by the investigation of Gudden and Pohl^" 

 on the behaviour of a compound Ca-Bi-Na phosphor. They found 

 that the maxima of photo-electric sensitivity occur at the same 

 regions of the spectrum as the maxima of the emission bands of 

 the phosphorescence. Unfortunately, a parallel experiment could 

 not be carried out with the diamonds, as the light emitted by their 

 fluorescence was much too weak to permit of spectral analysis. 



(d) Index of Refraction 



The statement of Gudden and Pohl,^^ that a substance is photo- 

 sensitive only in regions where its index of refraction is greater than 2, 

 is of interest. The fact that diamond has a refractive index of 2.4, 

 and has been found to be photo-sensitive, lends weight to this state- 

 ment. Refractive indices for a number of photo-sensitive substances, 

 as given in Groth's mineralogical tables, are found in the following 

 table : 



Substance Index of Refraction 



Diamond Nq. 2.41 



Zinc Sulphide Nd. 2.36 



Cinnabar (HgjS) Nq. 2.85 



Cuprite (CU2O) Nr. 2.85 



Wulfenite (PbMo04) Nr. 2.40 



Proustite (AgaAsSg) Nr. 2.97 



Willemite (ZnaSsOO No- 1.71 



Molybdenite (M02S3) Njnfra Red. 2.85 



The only substance in the above table which has a value less 

 than 2.3 for its index of refraction is willemite, which was not found 

 to be photo-sensitive. If, however, the quality of photo-sensitivity 

 depended on the refractive index of a substance, there is no reason why 

 one sample should possess it rather than another, and the cause of 

 photo-sensitivity can not be attributed to the high index of refraction. 



(e) Dark Current 



On application of high voltages to the diamond for a long time, 

 great irregularity manifested itself in the dark current in both direc- 

 tions. These irregularities are similar to those described above in the 

 photo-electric currents, and point to the development of counter 

 fields within the crystal. In fact, it seems clear that the irregularity 



loGudden and Pohl, Zeitschr. fur Physik, 3, p. 99, 1920; alâo 4, p. 206, 1921. 

 I'Gudden and Pohl, Phys. Zeitschr., Oct. 15, p. 535, 1921. 



