416 
have been treated systematically, the results of which are given here 
together. ; 
Ll. Electric double refraction in clouds of ammonium-chloride and 
other substances. 
Dispersion phenomena. 
2. Arrangement. The arrangement used for the investigation of 
clouds of ammonium chloride is essentially the same which has for- 
merly been used by Prof. Zeeman and the author. Fig. 1 represents 
it schematically. The light of the source V is rendered parallel by 
a lens L. This parallel beam then traverses successively the polari- 
zator P, a curved glass bar B, which serves as compensator, the 
condensator vessel with ammonium chloride, in which an electric 
field can be excited, and the analyzer A. The nicols are crossed, 
their polarization directions make angles of + 45° with those of 
the electric foree. Looking through the analyzer we see the neutral 
line of the bar as a black band in a light field. Double refraction 
of the ammonium chioride will now be revealed by a displacement 
of the band towards the place where it is compensated. Dichroism 
will weaken the band or cause it quite to vanish. 
Some particulars will be discussed in details. 
The ammonium chloride was contained in a glass condensator 
vessel nearly equal to that, which was described before.') For 
the measurements however it was altered in so far that the leaves 
of tinfoil which serve as condensator plates are applied to the inner 
side of the glass. 
The ammonium chloride was again formed in the vessel from 
NH, and HCl. 
For the electric field | could dispose of a Wimshurst machine 
and of two transtormators. 
The Wimshurst was driven by a three phase motor of about + 
1) ZEEMAN and HooGENBooM, Communication I. 
