On the Emission Lines of Solids. 317 



of the observations were photographic ; with the smaller 

 instrument the D lines appeared about 1 mm. apart, with the 

 larger instrument about 3 mm. The electric fields were 

 applied by means of a motor-driven Wimshurst machine, and 

 measured either by the equivalent spark-gap between brass 

 balls in air, or by an electrostatic voltmeter up to 10,000 

 volts. 



The observations were carried out at liquid-air temperatures 

 whenever possible, because of the greater sharpness of the 

 lines. For work in liquid air, the specimens were pressed 

 between two parallel brass plates about 12 mm. in diameter, 

 and held apart 2 mm. or 3 mm. by the specimen. A 1 mm. 

 hole through the centre of each plate served to transmit the 

 light in the absorption tests. In the fluorescence tests the 

 exciting light was focussed from the side, and the excited 

 lio-ht taken off through one of the holes. 



The first line examined was the strong sharp fluorescent 

 line X 5736, which is part of a rather complicated spectrum 

 emitted by a certain specimen of Weardale fluorite, studied 

 and described with great care by Morse *, through whose 

 kindness we had a specimen available. As this line is most 

 strongly excited by ultra-violet light, and there was no quartz 

 Dewar flask available, observations were made at room- 

 temperature only, in the case of the electric field. Tinfoil 

 electrodes were waxed on the opposite faces of the crystal ; 

 they were about 4'5 mm. in diameter and 13 mm. apart (the 

 thickness of the crystal), so that the field was by no means 

 uniform. However, the difficulty of insulating was con- 

 siderable, and this arrangement seemed good enough for a 

 qualitative test. Between these electrodes a potential 

 difference of about 50,000 v./cm. was maintained, the fluores- 

 cence transverse to the field being examined. The line was 

 excited by light from a heavy spark between mg. electrodes, 

 and appeared slightly wider than one of the D lines on the 

 plates, taken with the same slit-width. The electric field 

 produced no change whatever in the appearance of the 

 line. As is usual in such cases the fluorescent light is slightly 

 polarized ; but examination showed that the electric field 

 produced no change in the amount of polarization. 



In order to still further study the sensibility of this line 

 to external influence, it was examined for the longitudinal 

 Zeeman f effect, at liquid-air temperatures. For this purpose 

 the crystal was mounted in a cell in one end of a half-inch 



* H. W. Morse, Astrophys. Jonrn. xxi. p. 83 (1905). 

 t The Weiss magnet was very kindly loaned by Professor Richards, 

 of the Department of Physics, University of Pennsylvania. 



