CH.II.] HISTORICAL REMARKS 17 



excessive. Usually the charge is merely thrown into 

 oscillation, when the sphere is suddenly stopped ; and 

 it then emits a solitary wave or spherical shell of thick- 

 ness equal to the diameter of the sphere : or greater 

 than that diameter by the amount the sphere has 

 moved during its retardation. When the acceleration 

 is moderate, however, the radiation is less energetic 

 and also less intense : less energetic because its power 

 depends on the square of the acceleration, less intense 

 because it is spread over a thicker ethereal shell. 

 Rontgen rays are perceptible only when the speed was^ 

 great and the stoppage so sudden that the wave or 

 pulse- shell is strong and thin (see chap. viii.). The 

 thinner the pulses or wave shells the more penetrating 

 they are. If thin enough they could traverse matter 

 without affecting it or being affected by it. 



Historical Remarks. 



The doctrine of the behaviour of a charged sphere 

 in motion, and the calculation of the value of the 

 quasi inertia of an electric charge, was begun by 

 Professor J. J. Thomson in an epoch-making paper 

 published in the Philosophical Magazine for April, 

 1881 one of the most remarkable physical memoirs 

 of our time. 



The stimulus to this investigation was supplied by 

 those brilliant experiments of Crookes, published in 

 the Philosophical Transactions for 1879, which were 

 preceded by observations of Pliicker and Hittorf, and 

 related to other observations by Goldstein, Spottis- 

 wood and Moulton, and others, about the same period. 



In 1891 Sir William Crookes was President of 

 the Institution of Electrical Engineers, and in his 

 inaugural address he expounded further some of these 

 brilliant experimental investigations, to which Schuster 



L.E. 



