1912-13.] New Method of Starting Mercury Vapour Apparatus. 123 
15 seconds after the current was switched on. As an example of dura- 
bility, one lamp was found to work perfectly after a lapse of more than 
six months, although it had been in use from time to time during that 
period ; no deterioration of the vacuum had taken place. This showed, 
amongst other things, that the previously mentioned new method of sealing 
in the platinum wires is entirely satisfactory. 
It may easily be seen that this new method of starting mercury vapour 
apparatus does not depend on the form of the illuminating portion of the 
lamp, but may be applied without any difficu]ty to all forms of lamps, 
rectifiers, etc. We have found that the form of lamp specially described in 
this paper is extremely suitable for all kinds of spectroscopic work. One of 
the experimental Jena-glass lamps was successfully employed as a source of 
light in connection with a large concave diffraction grating spectrograph in 
use in the Natural Philosophy Department of the University of Glasgow. 
We desire to express our thanks to Professor A. Gray for the kind 
interest he has taken in the work connected with this paper. 
(. Issued separately April 11 , 1913 .) 
