some Problems of the Mass-Spectrogrrajrfi. 519 



existing element of each caustic. He gives an equation 

 (p. 94) for the position of the plate. 



This position had been already established by another 

 argument in the first paper cited, so that to this order of 

 accuracy Thomson's discussion agrees with ours. 



He then goes on to give a formula for the distance of the 

 image along the plate as a function of e/m, and points out 

 that this distance will not be a linear: function of the mass. 



Fig. 2. 



The order of approximation, however, which he uses is, we 

 think, inadequate for the purpose : the full analysis given 

 in the preceding sections is needed to obtain that true 

 relation. We have there seen that the observed distributions 

 are a good fit with the theory, and that the deviations from a 

 linear relation are very small. Such deviations as occur are 

 simply allowed for by the use of a calibration curve in 

 reducing the observations, so that criticism of the results 

 based on this non-linearity cannot be regarded as of great 

 weight. 



