1191 
possibly not be attainable at all’). This fear turned out to be 
unfounded. Temperatures of — 250° and — 251° do not procure 
any special difficulties. 
It was mentioned above, that the temperature in the experimental 
space could be kept constant to .01 of a degree during a long time. 
In confirmation of this we here reproduce two curves drawn according 
to the galvanometer-deflections of the two thermometers Pos and Pto;, 
representing the deviations of the temperature as a function of the 
time. They refer to an observation at — 244.°38. It must be men- 
tioned, however, that the management of the eryostat is far from 
easy or simple. A great deal of routine and experience is required 
to work the cryostat in such a way that the above constancy of 
the temperature is attained, and it is only after repeated vain efforts 
that even our highly trained technical staff has now learned to 
make the apparatus answer to the slightest hint. 
Much is due to Mr. G. J. Frim, chief mechanic of the cryogenic 
laboratory, under whose able guidance the apparatus was designed 
and constructed, and further to Messrs. L. and A. Ouwerkerk both 
attendants 1st class, under whose supervision the apparatus now 
works so excellently. 
1) Comp. Leiden Comm. No. 15la p. 4 note 1. 
