MECHANICS AND OPTICS. 229 



Unfortunately, the experiment is a very delicate 

 one, and cannot be performed successfully, except by 

 a physicist as skilful as Kaufmann. All suitable pre- 

 cautions have been taken, and one cannot well see 

 what objection can be brought. 



There is, nevertheless, one point to which I should 

 wish to call attention, and that is the measurement of 

 the electrostatic field, the measurement upon which 

 everything depends. This field was produced between 

 the two armatures of a condenser, and between these 

 two armatures an extremely perfect vacuum had to 

 be created in order to obtain complete isolation. The 

 difference in the potential of the two armatures was 

 then measured, and the field was obtained by dividing 

 this difference by the distance between the armatures. 

 This assumes that the field is uniform ; but is this 

 certain ? May it not be that there is a sudden drop 

 in the potential in the neighbourhood of one of the 

 armatures, of the negative armature, for instance? 

 There may be a difference in potential at the point 

 of contact between the metal and the vacuum, and it 

 may be that this difference is not the same on the 

 positive as on the negative side. What leads me to 

 think this is the electric valve effect between mercury 

 and vacuum. It would seem that we must at least 

 take into account the possibility of this occurring, 

 however slight the probability may be. 



VI. 



The Principle of Inertia. 



In the new Dynamics the Principle of Inertia is still 

 true — that is to say, that an isolated electron will have 



