458 



Prof. McLennan and Mr, Young on the 



was excited, for the voltage used was only a fraction of that 

 employed by Berndt in his experiments. 



In order to make certain that no chance impurities were 

 the cause of lines coming out which were observed in these 



~ o 



experiments below X=22Q0 A. U., different pieces of selenium 

 wt j re used as well as different carbon electrodes. The same 

 spectrum, however, was invariably obtained. Many plates 

 were taken, and four of the best of them w* j re used in 

 measuring up the wave-lengths of the lines. Twelve lines 

 in all were obtained and ascribed to the selenium spark. 



In determining the wave-length of the selenium lines the 

 following prominent aluminium *, zinc f, and cadmium t 

 lines were used :— 



Aluminium lines. 



Zinc lines. 



Cadmium lines 



X = 199057 A.U. 



X =2558-20 A.U. 



X = 2748-68 A.U 



35-90 „ 



0220 „ 



2573 15 „ 



1862-81 „ 



2138 66 „ 



2313-88 „ 



58-20 „ 



0006 ,. 



2288-12 „ 



54-80 „ 



2062-08 „ 



2265-04 „ 





2551 „ 



2194-71 „ 

 44-44 „ 



In fig. 2 (PL XII.) the first reproduction, "a," is the 

 spectrum of the aluminium spark in air, the second, " &," 

 that of the carbon spark in air, and the third, " c" that of 

 the combined carbon and selenium spark in air. 



In measuring up a plate the distances of the various 

 aluminium, zinc, cadmium, and selenium lines from the 

 aluminium line A, = 1854*8 A.U. were carefully measured 

 with a Hilger comparator. The distances of the aluminium, 

 zinc, and cadmium lines given above from the aluminium 

 line A. = 1854*8 A.U. were used as the ordinates of a cali- 

 bration curve and the wave-lengths of the lines as abscissae. 

 This calibration curve was then used to determine the wave- 

 lengths of the selenium lines. 



The results are probably accurate to one Angstrom unit. 

 The relative intensities of the lines were estimated by giving 

 the strongest line the arbitrary value 10 and referring to 

 this as the standard. The mean values of the measurements 

 of all the selenium spark-lines observed are given in 

 Table II. 



* Handke, Dissertation, Berlin, 1900, p. 18. 



f Eder and Valenta, Atlas Typischer Spektren t TVien. 



