876 
disks with a Wisuurst electrical machine, giving a tension of more 
than 90 kilovalt. A vacuum-glass was half filled with liquid air 
and covered with an ebonite stopper, which was perforated for the 
glass tubes. Though the moisture in the laboratory hardly ever ex- 
ceeded 30 to 40°/,, the whole apparatus was placed under the case 
of an exsiccator. In this way the ruby could for some time endure 
a tension of about 60 kilovolt, that is of 60/0,3 = 200 kilovolt/em. 
= 667 electrostatic units. 
The absorption lines, especially the two strongest lines A, and R, 
in the red (691,8 and 693,2 uu) were observed in the first order 
of a concave grating (radius 181 em., 5684 rulings per em.), which 
was mounted in the ordinary way. 1 mm corresponded fairly well 
to 1 gu, so that a change of the order of 0,005 uu could not escape 
observation. No influence of electrification could however be detected; 
in any case the displacement or separation is less than one hundredth 
part of the magnetic longitudinal effect in a field of 50 kilogauss. 
For the latter we formerly found the values: 
ee Fi 
Axis Il field; triplets with extreme separation dà: 0,37 0,48 uu 
Axis 1 field; quadruplets _,, =. ms dà: 0,62 0,62 uu. 
Probably an interferential method might give a sharper criterion 
than the rather small dissolving power of the grating. Such experiments 
might be made with the corresponding fluorescence spectrum of the 
ruby at a very low temperature; they must be delayed however 
until a more sunny season of the year. As this subject is of great 
importance it seems to me interesting even to determine the mini- 
mum limits in negative researches. 
I also made experiments with the neodymnitrate-hexahydrate 
(Nd(NO,),.6H,O) from the series of the rare earths. A natural 
monocline plate about 1 mm thick was cut 1 to one of the axes 
and in the manner above mentioned mounted between two very thin 
glass plates by means of Canada balsam. Observations were made 
at —-190° on the group of bands in the red, numbered from I to 
VIII in a former paper (loc. cit. § 32). On account of the smaller 
thickness of this slide the electric field was even stronger here than 
in the case of the ruby; but again no perceptible influence on the 
absorption bands was found. Now in a magnetic field the bands 
VII and VIII (676,6 and 677,2 uu) give doublets the separation of 
which reaches the largest observed value viz. dà =1,0 and 1,1 uw 
for 50 kilogauss. 
For the sake of completeness I repeated the experiment with an 
