618 Dr. F. W. Aston on the 



bend. The image on the plate will therefore be the part 

 of this figure falling on the narrow slot in X, X ; and as 

 the apparatus is not exactly symmetrical, its shape in the 

 spectra is the figure lying between the lines X, X in 

 fig. 4, c. 



Measurement of the Lines. 



The plates are measured against a standard Zeiss scale on 

 a comparator designed by the late Dr. Keith Lucas and 

 kindly lent by the Physiological Department. Some of 

 the very faint lines, although easily visible to the unaided 

 eye, were lost even with the lowest power eyepieces 

 obtainable. To measure these, an eyepiece giving 'a 

 magnification of about 2\ was designed by Dr. Hartridge 

 of King's College. 



The general method of deducing mass from position has 

 already been described (Phil. Mag. April 1920, p. 453). 

 Owing to some geometrical cause (probably analogous 

 to a caustic in optics), the more deflected edge of the line 

 is always the brighter and sharper, and it is the distance 

 of this from the register spot which is found to give the 

 most reliable values. For the highest accuracy, owing to 

 halation, one must only compare lines of approximately 

 equal intensity. As this edge is unfortunately not at right 

 angles to the spectrum, measurements can never be regarded 

 as absolute, unless extreme care is taken in the levelling of 

 the spectrum on the comparator. So although theoretically 

 it is sufficient to know the mass of one line to determine 

 (with the correction curve) those of all others, in practice 

 every effort is made to bracket any unknown line by 

 reference lines, and only to trust comparative measure- 

 ments when the lines are fairly close together. Under these 

 conditions the accuracy claimed for the instrument is about 

 one part in a thousand. 



Order of Results and Nomenclature. 



The various elements studied will be considered as far 

 as possible in the order in which the experiments were 

 performed. This order is of considerable importance, as 

 in most cases it was impossible to eliminate any element 

 used before the following one was introduced. Evacuation 

 and washing have little effect, as the gases appear to get 

 embedded in the surface of the discharge-bulb and are only 

 released very gradually by subsequent discharge. 



The problem of nomenclature became serious when the 

 very complex nature of the heavy elements was apparent, 



