423 
reduced to each other. The result of the measurements is given 
above. (See table p. 422). 
The results have also been represented graphically in two curves: 
fig. 6a for the lower, fig. 64 for the higher tensions. In the second 
diagram the tension is again expressed in kilovolts, in the first 
in volts. 
250 300 150 1000 
Fig. 6a. 
In the region of the lower tensions the curve shows a point of 
inflexion, so that it is possible — Vorer regards this as necessary — 
that by a first approximation the double refraction is proportional 
with the square of the field. 
