MASS SPECTRA OF THE ELEMENTS — ASTON. 



233 



The rays, after being magnetically deflected, pass between two ver- 

 tical brass plates, Z Z, about 3 millimeters apart, and finally reach 

 the photographic plate through a narrow slot 2 millimeters wide, 

 11.8 centimeters long, cut in the horizontal metal plate, X X. The 

 three brass plates forming a T-shaped girder are adjusted and locked 

 in position by a set of three leveling screws at each end; the right- 

 hand upper one is omitted in figure 4. The plates, Z Z, serve to pro- 

 tect the rays completely from any stray electric field, even that 

 caused by the photographic plate itself becoming charged, until 

 within a few millimeters of their point of impact. 



The photographic plate W, which is a 2-centimeter strip cut 

 lengthwise from a 5 by 4 plate, is supported at its ends on two 

 narrow transverse rails which raise it just clear of the plate, X X. 

 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 mecha- 

 nism, which is not shown in detail, is op- 

 erated by means of a torque rod V work- 

 ing 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 hy- 

 drogen lines with different magnetic fields, on a large plate placed 

 in the empty camera at a small inclination to the vertical. On de- 

 veloping this the actual track of the rays could be seen and the locus 

 of points of maximum concentration determined. The final adjust- 

 ment 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 leveling screws. 



Fig. 4. — The camera. 



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 onto the willemite screen Y, where 



