: 415 ‘ 
Physics. — “On the influence of an electric field on the light trans- 
mitted and dispersed by clouds.’ By C. M. HooeerBoow. 
(Communicated by Prof. ZprMaN.) 
(Communicated in the meeting of June 24 1916). 
1. J/ntroduction. The researches, which will be communicated 
here and which will be treated more completely in my dissertation, are 
the continuation of the experiments of Prof. Zeeman and HooGeNBoom, 
the results of which have been published already in these Proceedings ‘). 
They refer to the double refraction some clouds, especially of 
ammonium chloride, show in an electric field and to the influence 
of the electric field on the intensity of the transmitted and the 
dispersed light. 
Brocn *) has been the first to remark some action of the field on 
the propagation and the dispersion of light in and by a cloud of 
ammonium chloride. The details of this phenomenon have then been 
investigated by Prof. Zeeman and the author, by which research was 
found that such a cloud shows dichroism as well as double refrac- 
tion. As to the double refraction it was found that a recently formed 
cloud is positively double refracting, while after some time it beco- 
mes negatively double refracting. This change we then connected 
with the dimorphism of ammonium chloride, which after having 
been observed probably for the first time by Sras, has recently 
drawn more attention. 
Until now somewhat accurate measurements on the electric double 
refraction of ammonium chloride had not been made. This has been — 
done now and the results of this research have been given in the 
first part of this communication. Further the dispersion phenomena 
of the double refraction and the results reached with other nebulae 
will be discussed. 
In the second part those phenomena are discussed in which the 
field acts on the mtensity of the transmitted and the dispersed light. A 
first observation of this kind has been made by Brocn. The dichroism 
too belongs to this group of phenomena; further the authors had 
found already earlier, that in a direction perpendicular to the lines 
of force the transmitted natural light is weakened by the field. 
Now a more complete research has been made where all cases 
1) ZEEMAN and HooGeNBoom, These Proceedings XIV p. 558 and 786, XV p. 178. 
These papers will be referred to as Communication I, Il and Ill. See also Phys. 
Zs. 18, 913, 1912. 
*) Broen, G R. 146. 970, 1908. 
