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were to enter the chamber through the sHt they all would follow the 

 same semicircle and assemble on the very same spot on the plate, 

 the distance of which spot from the slit would tell the observer their 

 mass. But when a beam of ions of a single element is projected 

 through the slit, it is not usually a single spot which appears upon 

 the plate. All students of physics have seen reproductions of such 

 plates, chiefly from Aston 's magnificently ample store. I reproduce 



Fig. 3 — Scheme of Bainbridge's apparatus for accurate measurement of the masses 



of isotopes. 



here two from Bainbridge's, Fig. 4 for zinc and Fig. 5 for germanium. 

 These are "mass-spectra" every spot or "line" of which is the evidence 

 of a separate isotope of the element in question. Germanium and 

 zinc are neither the least nor the most profuse in isotopes among the 

 elements; there are still a few (fluorine and sodium, for instance) for 

 which only one has been discovered, and at the other extreme there is 

 tin with no fewer than eleven. 



It is, of course, the charge-to-mass ratio of the ion rather than its 

 mass which is deduced from the position of the spot and the strengths 

 of the accelerating and deflecting fields. (There is no need of giving 

 the formula here, as it is to be found in every textbook and is readily 



