242 THE ROYAL SOCIETY OF CANADA 



The crystal was securely mounted in a holder between two flat 

 electrodes faced with tin foil. Small springs kept the electrodes 

 pressed against the crystal, thus insuring good contact. The holder 

 was fixed at the exit slit of the spectrometer, whose calibrated drum 

 enabled any desired wave-length of light to be focussed on the crystal. 

 The light fell on the crystal with perpendicular incidence, and when 

 not desired could be cut ofï by a shutter. 



A set of storage batteries allowed fields up to 11,500 volts per 

 cm. to be applied to the crystal. 



To measure the photo-electric current, a sensitive Broca iron- 

 clad galvanometer was used. Its senitivity was 2 X 10"^ amps, 

 per mm. deflection with scale distance of 1 meter, and its period was 

 26 seconds. To protect the galvanometer, a water resistance was 

 kept in series with it. 



The energy in the radiation from the lines of the spectrum was 

 measured by a sensitive bismuth-silver thermopile, placed at the exit 

 slit of the spectrometer. Measurements of the thermo-electric cur- 

 rent were made with the same galvanometer, which was sensitive 

 enough to register the effect of radiation as far down as X = 2400 A. 

 Although great care was taken to keep the current through the lamp 

 steady, it was found that energy measurements varied considerably. 

 For that reason, energy measurements were repeated before each set 

 of readings for photo-electric effect, and the flnal values used were 

 the averages of four readings. 



The arrangement of the apparatus is shown in Fig. 6. 



3. Properties of the Crystals. 



(a) Description. 



1. Diamond No. 1 was in the shape of an approximately rect- 

 angular plate, about 7 mms. by 5 mms., the thickness varying from 

 .75 mms. at one end to 1.5 mms. at the other. One of the large faces 

 was quite plane, the difference in thickness being occasioned by a 

 curvature on the other large face. It was a natural crystal, quite 

 colourless and semi-transparent. 



2. Diamond No. 2 was an irregularly shaped plate, roughly 6 

 by 4 by 1 mms. It was formed by tne growth together of two crys- 

 tals, and contained a large, black occlusion. It also was a natural 

 crystal, whose colour was not as clear as that of diamond No. 1. 



3 Diamond No. 3 was an oval plate, 11 by 9 by H mms., 

 beautifully cut and polished. It was quite colourless and transparent. 



