Constitution of Atmospheric Neon. 453 



They are important lines of reference and are certainly of 

 the relative masses given above to the order of accuracy 

 (one-tenth per cent.) claimed in the present experiment. 



In ATY. the deflexion has been still further increased and 

 a new group of lines, the C 2 group 24, 25, 26, 27, 28, 29, 30 

 containing the strong reference line of CO (or C 2 H 4 ), have 

 come into view. In A III. of the 2 group only 15 and 16 

 are visible, and in A V. the C 2 group has moved to the left 

 and the strong line 44, C0 2 is seen to the right. 



Plate B was taken with CO to which about 20 per cent, 

 of atmospheric neon had been added. Considering the 

 spectrum B III. it will be seen that four unmistakably new 

 lines have made their appearance, one pair between the C\ 

 and C 2 groups, another weaker pair to the left of the C x 

 group. The first pair are (Ne a ) + 20 and (Ne*) + 22 singly 

 charged, the second pair are the same atoms with double 

 charges 10(Ne a ) ++ and ll(Xe^) + + respectively. The other 

 spectra consist of lines already mentioned brought into 

 different positions to increase the convenience and accuracy 

 of comparison and, in addition, there are on CI. two other 

 valuable reference lines, ++ apparent mass 8, and on the 

 extreme left just visible C ++ apparent mass G. 



Method of comparing masses. 



It will be noticed that although the lines are broad (the 

 best focus was only obtained by a series of trials after these 

 results were completed) their edges, particularly their left- 

 hand edges, are remarkably sharp, so that measurements of 

 a reasonably good line from the register spot repeat to a 

 twentieth of a millimetre with certainty. Hence for accurate 

 determination of unknown lines only two assumptions need 

 be made. Firstly, that the masses of the reference lines are 

 known, and secondly that, whatever the function connecting 

 displacement with mass, any two positions on the spectrum 

 being taken, the ratio of any two masses giving lines in 

 these positions will be constant. This being so, by moving a 

 group of reference lines into overlapping positions along the 

 spectrum it is clear that the whole length can be plotted 

 out and calibrated. 



Fortunately there is an easy method of testing both these 

 assumptions, for although it is impossible to measure the 

 magnetic field to one-tenth per cent., it can be kept constant 

 to that accuracy while the electric field is altered by a 

 known ratio. But, for constant deflexions, mv 2 v: X and 

 mi: a H = const. Therefore m ^ X -1 , so that, to take a 



