﻿418 M~ A. Wiillner on the Spectra of 



same dimensions as had been used for" the experiments with hy- 

 drogen, the density of the gas had to be much smaller to allow 

 the current of the same induction-coil to pass through when the 

 same number of elements were used. While hydrogen allowed 

 the current to pass even under a pressure of 135 millims., several 

 experiments showed that with oxygen continuous passage only 

 took place when the pressure was diminished to 45 or 47 millims. 

 The light is whitish, but far too weak for a spectrum-investiga- 

 tion ; this was only possible when the density was diminished to 

 28 or 30 millims. The light appears even then whitish-coloured. 

 The spectrum presents six bright lines — a red line (the flesh-red 

 one characteristic of oxygen and designated by Pliieker 0«), 

 two green and two blue lines, and a violet one. The brightest 

 have then the following positions of least deviation : — 



a, the flesh-red line ... 61 54 30 

 The second green one . . . 63 28 

 The violet 67 20 



After a longer passage of the current, the second red line in- 

 dicated by Pliieker is recognized. 



When the pressure is diminished to 25 millims. the tube still 

 shines with a whitish light, the brightness of which, however, has 

 greatly increased. Besides those previously observed, there are 

 seen in the spectrum- three fainter green lines, at about 64° least 

 deviation. Under a pressure of 18 to 20 millims. the colour of 

 the light somewhat passes into violet, the brightness is increased ; 

 there appear besides the former lines two orange-coloured ones, 

 two very faint yellowish green, a faint blue, at about 65° 10', 

 and a faint violet line at the end of the spectrum. 



The pressure being diminished to 6 millims., some new lines 

 occur — a bluish green, and a violet which is somewhat further 

 deflected than those previously mentioned. 



At the same time the background on which the lines stand 

 out no longer appears quite dark, but here and there distinctly 

 continuously illuminated. Such a continuous field probably 

 forms the background against which the previously mentioned 

 three faint green lines at about 64° are formed. 



Without these lines disappearing, the continuously illuminated 

 fields stand out more distinctly when the pressure is further .di- 

 minished ; and when it only amounts to fractions of a milli- 

 metre, the background of the spectrum has become changed into 

 a spectrum of the first order, consisting of several beautifully 

 shaded fields. The colour of the light has become more green ; 

 and in accordance with this the continuously illuminated parts 

 lie in the green and in the blue. 



The first, very faint yellowish part appears just on the right 



