1023 



various checking measurements, some of which made with a mercury 

 arc lamp we shall detail here '). The arrangement of (he api)aratus 



is' shown in fig. 2. 



VG 



.3peclrop72olgnielsr_ __ k 



1^ 



Fig. 2. 



Under the mercui'v lamp a plate of ground-glass was introduced. 

 A second similar plate B was illuminated by A. The illumination 

 of B was completely uniform. An image of B was thrown upon 

 a 3"f^ ground glass plate Z) by means of a lens C. The slit in the 

 HiLGER-quartz spectrograph was completely uniformly illuminated by 

 Z). The sector was placed close in front of E. At G a mirror was 

 placed which could be removed. By means of a spectro-photometer 

 of the König-Martens type the illumination of D could be measured 

 in the light of the green mercury line. 



The measurements proceeded as follows. When the mercury 

 lamp had become constant, the illumination of D was determined 

 by the speclro-photometer. Then the mirror G was removed and 

 the plate illuminated for three minutes. After this D was again 

 measured with the photometei-. The illumination was then changed 

 by reducing the lens C with a diaphragm. The measurements were 

 repeated and a new photograph taken. 



In this way we made several series of readings with different 

 intensities. 



The differences in length of the spectral lines were measured by 

 placing the negatives on top of each other and shifting them until 



') Other checking measurements carried out previously may be found in Diss, 

 Hamburger p. 87, 91, 02, 96, 108 and 113. 



71* 



