462 Prof. McLennan and Mr. Ireton on Fundamental 



and shorter in wave-length as the speed of the electrons was 

 increased. In particular, experiments were directed to this 

 end with mercury vapour, but it was found that when the 

 electrons were given kinetic energy corresponding to 4*9 volts, 

 the radiation of wave-length A = 2536*72 flashed out; and 

 then, as the velocity of the electrons Avas increased no photo- 

 graphic record was obtained of any wave-lengths shorter 

 than \— 2536'72 A.U., until the velocity corresponding to 

 102 volts was reached, when the arc struck and the many- 

 lined spectra came out. 



The question has, however, been re-examined by Bergen 

 Davis and Goucher *, and in a series of brilliantly designed 

 experiments in which the photoelectric effect was used for 

 detecting the existence of particular radiations, they have 

 shown that when mercury vapour of very low density was bom- 

 barded by electrons, radiation of wave-length A, = 2 53 6* 72 A. U. 

 was emitted without ionization at an impact voltage of 4*9 

 volts, and that when the impact voltage was increased to 

 6'7 volts a radiation of wave-length \=1849 A.U. came out 

 as well. With still higher impact voltages no additional 

 types of radiation appeared before ionization of the vapour 

 occurred, which took place with an impact of voltage of about 

 10-4 volts. 



With a view to confirming this result by the photographic 

 method the original experiments of one of us have been 

 repeated by the writers and extended to include vapours 

 other than mercury. The following paper contains an 

 account of these experiments. 



II. Experiments. 



In carrying out the experiments, the form of vacuum arc- 

 lamp used is shown in fig. 1, similar to the one described 

 by McLennan and Henderson f. It consisted of a tube of 

 fused quarts possessing three arms, B,, S, and M N, and a 

 receptacle L. Some of the metal to be used in producing 

 the vapour was placed in the receptacle L. The arms were 

 about 40 cm. long, so that when the receptacle was heated 

 all wax joints remained quite cool. A short piece of tungsten 

 was attached to two wires which constituted a heating circuit, 

 these being passed through an ebonite plug and sealed in at A. 

 A short iron tube, in which was sealed a crystal quartz plate, 



* Bergen Davis and Goucher, Phys. Rev. vol. x. no. 2, p. 101. 

 f McLennan and Henderson, loc. cit. 



