CONTEMPORARY ADVANCES IN PHYSICS 309 



mentioned (omitting the work of a few students on a special question, 

 the relative abundance of the isotopes of lithium, and that of Bleakney 

 on the isotopes of hydrogen and neon). Outstanding, and for years 

 unique, is that of Aston of the Cavendish Laboratory, who took over 

 the problem from Thomson and has bound up his name with isotopes 

 by fourteen undeviating years of concentration. There are two 

 stages of the post-war history: the period when isotopes were merely 

 counted and their masses roughly estimated, and the period (in the 

 midst of which we now stand) when their masses are measured with 

 precision rivalling the vaunted accuracy of the chemical atomic 

 weights, and also their relative proportions or "abundances" in the 

 mixtures which we usually call elements. Aston initiated both these 

 periods. In the earlier of them Dempster, who also had been trained 

 before the war in the analysis of ions, separated several of the elements 

 into their isotopes. Costa made a couple of very accurate measure- 

 ments of mass, but then abandoned the field. Bainbridge entered it 

 after the second period commenced, and is now measuring atomic 

 masses with an exactitude equalling Aston's. 



In Aston's apparatus the deflecting fields are disposed in an intricate 

 and ingenious way, so that ions of equal mass shall be brought to the 

 same point on the photographic plate even though their speeds be 

 far from equal. This is because he usually derives his ions from a self- 

 sustaining glow-discharge, of which the electric field serves as his 

 accelerating field and imprints different speeds upon different ions of 

 equal mass because they start from different places in the discharge. 

 Much simpler are the schemes of Dempster and of Bainbridge, in which 

 the sole deflecting agency is a uniform magnetic field, which swings 

 the ions around in semicircles from the slit where they enter the 

 deflection-chamber to the plate on which they impinge (Fig. 3). 

 This, however, does not work properly unless all the ions of a particular 

 mass have very nearly the same velocity, so that either they must 

 leave the source with very low speeds and be subjected to the same and 

 relatively large accelerating voltage (such was the case in Dempster's 

 work) or else there must be some device for preventing all ions but 

 those of a very narrow velocity-range from reaching the deflection- 

 chamber. Bainbridge's device for this latter purpose is shown in 

 Fig. 3; between the plates of the "velocity-filter" a transverse electric 

 field is superposed on the magnetic field which is at right angles to 

 the plane of the paper, and no charged particle gets through to the 

 slit unless its speed is very nearly equal to the ratio of the field- 

 strengths. 



If a beam of ions all of identical mass M and charge e and speed v 



