360 Prof. J. J. Thomson on 



pointed out, in accordance with that view; and direct mea- 

 surements have shown thai the intensity of the rays in 

 different gases is not connected with the amount of hydrogen 

 in the tube. The method adopted was to measure photo- 

 metrically the brightness of the phosphorescent patches 

 produced by the rays with the aforesaid values of e/m on a 

 screen covered with willemite. This was done as follows: — 

 Light of the same colour as the willemite phosphorescence 

 was produced by sending the light from an incandescent 

 lamp through a weak solution of fluorescine : with a little 

 care it is easy to get a solution in which the fluorescence 

 produced by white light is a very good match for the fluor- 

 escence produced by the positive rays on the willemite. A 

 small glass tube A (fig. 1) containing such a solution was 



placed i against the willemite screen on which the rays 

 impinged ; the tube was illuminated by light whose intensity 

 was adjusted in the following way. A Nernst lamp was 

 placed at the end of a long tube B ; the light from this, after 

 passing through a lens, went through two Nicol prisms N, N 

 placed in graduated holders : after passing through the nicols 

 the light w^as reflected parallel to the screen from a mirror M 

 and produced in the fluorescine a phosphorescent patch side 

 by side with that produced by the rays on the willemite. 

 When the planes of the two nicols were at right angles to 

 each other, no light got through ; when the planes were 

 parallel so much light got through that the phosphorescence 

 of the fluorescine was greater than that of the willemite. 

 By rotating the nicols the angle between them could be 

 adjusted so that the light passing through made the phos- 

 phorescence of the fluorescine equal to that of the willemite. 

 The angle between the planes of the nicols in this position 

 gave a measure of the intensity of the light from the fluores- 

 cine, and therefore of that produced by the positive rays. 



The current passing through the discharge-tube was mea- 

 sured by a galvanometer. The brightness of the positive rays 

 depends upon a good many things besides the current through 

 the tube; but the current is one of the important factors, and 



