COBLENTZ AND EMERSON: PHOTOELECTRIC SENSITIVITY 529 



into numerous fine needle-like crystals. The first sample ex- 

 amined was a small mass of crystals (size 1 by 1 by 0.7 mm.) 

 held by compression between two heavy electrodes of copper. 

 When the crystal was exposed to the standard carbon lamp no 

 change in conductivity could be detected with certainty. 



A second sample, 3 by 6 by 1 mm., had the copper wire ter- 

 minals attached by fusion, as already described. The e.m.f's 

 applied were the same as for the preceding sample. When ex- 

 posed to the standard lamp no change in conductivity was ob- 

 served. These results being contradictory to those published 

 by Case 6 who used a three stage audion amplifier to detect 

 the change in conductivity of the crystals, the foregoing experi- 

 ments were repeated in the manner described by him. For 

 this purpose the light from an acetylene flame shining through 

 a slit 2 by 10 mm. was focused upon the crystal by means 

 of a triple achromatic lens, 6 cm. in diameter and 18 cm. focal 

 length. The light was interrupted by means of a sectored disk 

 having 15 openings and operated by means of an electric motor, 

 the speed of which could be varied. The usual speed gave 240 

 interruptions per second. The crystal was connected to a three 

 stage audion amplifier and telephone receiver. A crystal of sele- 

 nium or a selenium cell produced a loud note, but the samples 

 of boulangerite and jamesonite, which by previous tests were 

 light-sensitive, did not give a musical sound in the telephone. 



The sample of bismuthinite with electrodes sealed on produced 

 no audible note when exposed to light. 



At least a dozen samples of bismuthinite held by compression 

 between heavy copper electrodes were examined in connection 

 with the amplifier. Of this number only two samples appeared 

 to be light-sensitive. One sample produced only a faint sound 

 in the telephone receiver. The second sample produced a loud 

 note in the telephone. The sound was the loudest when the 

 crystal was exposed along the line of contact with the copper 

 electrode. Covering the crystal with red glass did not reduce 

 the loudness of the note very much, indicating that the effect is 

 due to heating of the material. Unfortunately, this crystal was 



6 Loc. cit. 



