712 Dr. F. W. Aston on a 



Referred to axes 0X ? OY the focus is at rcos (<£ — 20')^ 

 r sin (<j) — 26'), or r, b . 26'; so that to a first-order approximation, 

 whatever the fields, so long as the position of the diaphragm 

 is fixed, the foci will all lie on the straight line Z F drawn 

 through Z parallel to OX. For purposes of construction Gr 

 the image of Z in Y is a convenient reference point, <b being 

 here equal to AO 1 . It is clear that a photographic plate, indi- 

 cated by the thick line, will be in fair focus for values of ejm 

 over a range large enough for accurate comparison of 

 masses. 



The arrangement, which has a distinct resemblance to the 

 ordinary quartz spectrograph, gives very complete control. 

 The field between the plates can be adjusted to allow the 

 brightest part of the electric spectrum to be used which, as 

 has been shown, is in general the same for all normal rays 

 under steady discharge, and the values of e/m can be com- 

 pared very accurately from the positions of their lines relative 

 to those of standard elements which can be brought to any 

 desired position on the plate by varying the magnetic field 

 strength. 



Preliminary results. 



In order to test the method before making the somewhat 

 elaborate camera, a temporary apparatus was set up using an 

 existing camera, the plate being in the position indicated by 

 the dotted line AB. Under these conditions the focus can 

 only be good at or near the point B. The results so far 

 obtained are exceedingly promising, and show that as far as 

 intensity and sharpness of the lines are concerned no serious 

 difficulty need be apprehended. Plate IX. fig. 1 is a photo- 

 graph taken with an electric field but with no current 

 passing through the magnet. It shows the undeflected spot 

 as a sharp bright line — with a patch of fog above it due to 

 some internally reflected light — and the electric spectrum 

 of the positive rays spread out below. In fig. 2 the magnet 

 has been turned on, other conditions being identical with 

 fig. 1. It will be seen that although there is no diaphragm 

 in this apparatus and therefore practically the whole of the 

 electric spectrum is in use, yet the rays corresponding to 

 the hydrogen molecule are concentrated as a sharp bright 

 line 1*4 cm. above the undeflected spot and displaced a little 

 to the right as the magnetic pole-pieces are not set truly 

 vertical. Fig. 3 was taken with a smaller field, showing 

 the hydrogen atom near the focal point and the molecule 

 below it very much out of focus. 



Plate IX. fig. 4 was taken with a much larger magnetic 



