616 Dr. F. W. Aston on the 



lengthwise from a 5x4 plate, is supported at its ends on 

 two narrow transverse rails which raise it just clear of 

 the plate XX. Normally it lies to the right of the slot 

 as indicated, and to make an exposure it is moved parallel 

 to itself over the slot by means of a sort of double lazy- 

 tongs carrying wire claws which bracket the ends of the 

 plate as shown. This mechanism, which is not shown in 

 detail, is operated by means of a torque rod V working 

 through a ground glass joint. Y is a small willemite 

 screen. 



The adjustment of the plate-holder so that the sensitized 

 surface should be at the best focal plane was done by taking 

 a series of exposures of the bright hydrogen lines with 

 different magnetic fields on a large plate placed in the 

 empty camera at a small inclination to the vertical. On 

 developing this, the actual track of the rays could be seen 

 and the locus of points of maximum concentration deter- 

 mined. The final adjustment was made by trial and error 

 and was exceedingly tedious, as air had to be admitted and 

 a new plate inserted after each tentative small alteration of 

 the levelling-screws. 



Experimental procedure. 



The plate having been dried in a high vacuum over- 

 night, the whole apparatus is exhausted as completely as 

 possible by the pump with the stopcock L open. I x and I 2 

 are then cut off from the pump by stopcocks and immersed 

 in liquid air for an hour or so. The electric field, which 

 may range from 200 to 500 volts, is then applied and 

 a small current passed through the magnet sufficient 

 to bring the bright hydrogen molecule spot on to the 

 willemite screen Y, where it can be inspected through 

 the plate-glass back of the cap P. In the meantime the 

 leak, pump, and coil have all been started to get the bulb 

 into the desired state. 



As soon as this is obtained and has become steady, J 2 is 

 earthed to prevent any rays reaching the camera when the 

 plate is moved over the slot to its first position, which is 

 judged by inspection through P with a non-actinic lamp. 

 The magnet current having been set to the particular value 

 desired and the diaphragm adjusted, the coil is momentarily 

 interrupted while J L is raised to the desired potential, after 

 which the exposure starts. During this, preferably both at 

 the beginning and the end, light from a lamp T is admitted 

 for a few seconds down the tube R (fig. 1) the ends of 



