464 Prof. McLennan and Mr. Ireton on Fundamental 



shows the single line A == 2536'72A.U. obtained with an impact 

 voltage of 6 1 volts. Although Schumann plates were used, 

 exposures as long as 10 hours, with an impact voltage of from 

 8 to 8*5 volts, failed to give any trace of the line X= 1849 '6 A.U. 

 In view of the results obtained by Bergen Davis and Goucher, 

 we can only conclude that in all our experiments the density 

 of the vapour used was too great. It is known that radiation 

 of wave-length X = 1849*6A.U. is strongly absorbed and 

 scattered by mercury vapour even of low density, and it is 

 possible that this accounts for the non-appearance of any line 

 at this wave-length. 



(b) Zinc. 



With zinc much better results were obtained. When the 

 vapour of this metal was bombarded with electrons whose 

 impact voltage was about 4 volts, monochromatic radiation 

 of wave-length X = 3075"99 A.U. was recorded. As the 

 impact voltage was increased no additional indication was 

 observed until about 6 volts was reached, when the line cor- 

 responding to \ = 2139 33 A.U came out on the plates. 

 Reproduction No. 1, fig. III. shows the many-lined spectrum 

 of the zinc spark. o Reproduction No. 2, rig. III. shows the 

 line \ — 3076-00 A.U., which was brought out when the 

 impact voltage was 5*6 volts. The plate for this spectrogram 

 showed in addition the line corresponding to \ = 3260* 17 A.U. , 

 which would indicate, since the line was extremely faint, that 

 a trace of cadmium was present as an impurity in the zinc. 

 Reproduction No. 3 shows both the lines corresponding to 

 X = 3075'99 A.U. and X = 2139°33 A.U., and was obtained 

 with an impact voltage of 7'5 volts. 



(c) Cadmium. 



With cadmium vapour, results were obtained similar in 

 character to those recorded with zinc. No photographic 

 record was obtained of any radiations until an impact voltage 

 of about 4 volts was reached. Under these circumstances, 

 the line at wave-length \ = 3260*17 A.U. came out on the 

 plates. With still higher impact voltages no additional 

 radiation appeared until an impact voltage slightly less than 

 6 volts was obtained. With this impact voltage the line at 

 \ = 2288*29 came out on the plates in addition to the line 

 at\ = 3260-17A.U. 



Reproduction No. 1, fig. IV. is that of the many-lined 

 spectrum of the o cadmium spark. No. 2 shows the single line 

 at X = 3260"17 A.U., and was obtained with an impact voltage 



