﻿410 M. A. Wiillner on the Spectra of 



being gradually exhausted, the pressure of the enclosed gas being- 

 read off on the mercui'ial column of the pump. 



In this way it was found that the current of the small induc- 

 tion-coil can, under a pressure of 135 millims., strike across the 

 tubes, the electrodes of which are 1*4 decim. apart; the tube 

 shines with a white light, the intensity of which is too small to 

 analyze it prismatically. 



When the pressure of the enclosed gas is diminished, the 

 brightness of the light steadily increases, and under a pressure 

 of 100 millims. it is bright enough to be investigated with the 

 prism. Under this pressure the light of the tube is bluish 

 white, and sometimes it appears for a moment reddish-tinged. 

 It gives a continuous spectrum, upon which, when the light ap- 

 pears reddish, the bright lines H a. and H ft distinctly stand out. 



Under a pressure of 70 millims. the light of the gas is reddish 

 white, and in the spectrometer the continuous hydrogen-spec- 

 trum is seen as above described. It shows first of all Ha; as 

 far as half the distance between H a and D the field of view is 

 dark ; then follows a reddish-yellow to a greenish-yellow part, 

 consisting of a series of beautifully shaded bands; then another 

 dark one, and then as far as H ft a feebly bright green field, on 

 which four brighter green bands stand out. Behind H ft, as far 

 as H 7, the field of view is illuminated by a feeble blue light, in 

 which two bands stand out more brightly — the first at 0*3 of 

 the distance H ft to H 7 behind H ft, the second in the middle 

 between these lines. 



If the pressure be further diminished, the light in the tube 

 increases in brightness, the colour becomes redder, and the spec- 

 trum continually more beautiful. Even under a pressure of 52 

 millims. the spectrum is developed just as has been already de- 

 scribed ; it becomes brighter and more beautiful as the gas is 

 still further rarefied, until the pressure amounts to about 30 mil- 

 lims., at which the spectrum appears extremely brilliant. 



On further rarefaction the brightness of the continuous spec- 

 trum decreases, while the three hydrogen-lines become continually 

 brighter. Under a pressure of 21 millims. the lines II a, H ft, 

 H 7 were very beautiful, the reddish«yellow part of the continuous 

 spectrum also beautiful, but the green part of the spectrum much 

 enfeebled, only the bright bands in them being visible. Near 

 H ft, towards the more refrangible side, only the two brightest 

 lines could be recognized. 



Under a pressure of 10 millims. the reddish-yellow part could 

 still be seen in its brightest bands ; in the green, individual 

 bands were still faintly indicated ; beyond H ft scarcely a band 

 was visible. 



Under a pressure of 6 millims, the reddish-yellow part was just 



