( 249 ) 
Henceforth I will denote by q the theoretical value of the limit 
of resolution calculated according to (1), by 9 the experimental 
value. By means of a Horrmann direct vision spectroscope the 
light of the vacuum tubes (driven by a RumMkKorrrF) undergoes the 
necessary preliminary analysis. In some cases absorbing media were 
therefore sufficient. In some experiments the mercury arc-lamp of 
Faspry and Peror was used. 
§ 2. The very intense green (5460) line of mercury was inves- 
tigated first. Using the echelon in a position in which two strong 
lines of equal intensity corresponding to successive orders of the 
radiation were visible, I could distinguish also 5 faint, very narrow 
lines between the principal ones. The distance between two pairs 
of these lines was very small. 
As I could not find a table of the wave-lengths of these feeble radia- 
tions, I addressed myself to Messrs FABRY and Perot. [am very much 
obliged to Messrs Perot and Fasry for their kindness to investi- 
gate for me anew the green radiation of the mercury arc in vacuo. 
The following scheme represents the constitution of this very 
complex radiation according to their observations. The ordinates 
are approximately proportional to the intensities. 
-¥/ -14 ~9,5 Otls +15 #25 
The given numbers are only approximate, especially (— 14) 
and (— 9,5). 
The radiation (+ 1,5) was observed by Fasry and PerOT only 
in the radiation of a MIcHELSON tube; it is too approximate to the 
principal radiation to be seen separately in the are light. In the 
photographical reproduction in the Astrophysical Journal *) of the 
interference fringes of the green mercury line the radiation (— 41) 
evincides with the radiation (+ 15) and is therefore invisible. 
I could distinguish very clearly the radiations (— 9,5) and (— 14) 
1) Fapry and Peror Astrophysical Journal. Vol. 13. p. 272. 1901. neh 
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