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times the normal width of the slit *) for the arrangement used. As 
lens [ used a double lens of quartz-calcium fluorite, transparent to 
ultraviolet rays, and moreover achromatic. The image of the source 
of light always fell at the same place of the slit. 
It was generally necessary to separate the vertical and horizontal 
components with a calespar rhombohedron, placed between the 
source of lignt and the lens. One of the two images formed in this 
way, was projected on the slit. 
The magnet was a large-size Du Bois magnet, of which the end- 
planes of the conic pole tops were circular with a section of 
generally 8 mm. The distance between the poles was never greater 
than 4 mm. The magnetizing current was measured with a thermic 
Ampèremeter of Hartmann and Braun, and generally amounted to 
15 Amperes. 
As source of light was used a spark between electrodes 
of the metal under examination, or alloys of it. The spark was, 
parallel to a condenser, in the secondary circuit of a transformator 
(of Kocm and Srerzer at Dresden), of which the primary circuit was 
fed by the municipal alternating current. Besides in the secondary 
circuit there was found an auxiliary spark, and an adjustable self- 
induction, in series with the spark. The particulars of the spark 
discharge (intensity of light, sharpness of the spectrum lines formed, 
melting of the electrodes) and the influence on this of self-induction, 
capacity and auxiliary spark will be fully discussed in my Thesis 
for the Doctorate. 
As photographic plates I mostly used ‘Agfa Röntgen” plates, 
which | made previously sensitive to colour by bathing them in 
“Pynachrom.” The plates prepared in this way were preferable in 
my opinion to the commercial colour-sensitive plates. 
The measurements were performed with a Zeiss comparator. The 
results were divided into four groups according to their greater or 
less accuracy, which I gave in the tables the weight: 4, 3, 2, 1. 
The first group had a _ probable error of 1°/, or less, the second 
from 1°/, to 5°/,, the third from 5°/, tot 10°/,, the fourth of more 
than 10°/,. For the precautions taken in the measurements and the 
determination of the error I refer to my Thesis. 
Also the preliminary experiments: the focussing of the plate-holder, 
the determination of the dissolving power, and the scalar value will 
not be treated further here, but may be found in my Thesis. The 
resolving power was certainly not smaller than the theoretical. 
') SonusreR. The Optics of the Spectroscope. Astroph. J., 21, 197, 1905. 
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