Fig. 16. 
432 
Molecular Physics in High Vacua. [J une, 
in the middle of the tube (Fig. 17, a). At the side of the 
tube is a small eledtro-magnet, which I can set in 
adtion by touching a key, and the focus is then drawn to 
the side of the glass tube (Fig. 17, b). To show the 
