418 
forms part of the secondary circuit. By first shutting the switch 5, 
and then S,, we obtain in a simple way between the ends of the 
secondary circuit very different potential differences soon after another 
at least if the resistances W, and W, are very different from each 
other. 
Electrometers of Braun were used to measure the tension. They 
had been calibrated before. The curved glass bar was again very 
useful as a sensitive compensator by means of which differences in 
phase of 6 > 10-74 may still be detected, as bas been found formerly. 
As a source of light in some cases a Nernst lamp was used, 
generally however an are lamp. 
3, Measurements of the electric double refraction of a cloud of 
ammonium chloride. | 
By means of the arrangement described in the preceding paragraph, 
the phenomena could be investigated with the naked eye quali- 
tatively only. Therefore a camera or a telescope with an ocular- 
micrometer were used for the measurements. Both were inserted 
behind the analyzer (see fig. 1). 
In order that the place of the black band could be determined, 
two horizontal wires were stretched on the bead, one horizontally 
and one vertically. Along the latter the distances 
could be measured. The image, formed by the lens of 
the camera or the objective of the telescope, is repre- 
sented by fig. 4. : determines the place of the band. 
If now in the path of the light a double refracting 
substance is inserted with its principal directions 
2 5 a as 
parallel to those of the bead, this ratio becomes —. From the theory 
) . 
of the bead it now follows, that mie proportional to the difference 
b 
in index of refraction of the ordinary and the extra-ordinary beam 
(ny—m»). In the case of the photographie method the values « and 5 
were measured on the plates. In the observations by means of the 
telescope the ocular micrometer with a seale division of five per 
mm. enabled to read the places directly, from which thus the 
displacements which were proportional to (n,— 7) could again be 
derived easily. 
A difficulty of the measurements was the change the same cloud 
undergoes in the course of time. These changes are of different 
nature: 1. the particles, by which the nebulum is formed, gra- 
