On Rays of Positive Electricity. 359 



1/3400 de millimetre. Un grand nombre de stries avaient 

 traverse la couche d'argent de maniere a donner naissance 

 a autant de fentes d'une tenuite extreme. Ces lignes lumi- 

 neuses etant observees, a l'aide d'un analyseur, an microscope 

 eclaire par la lumiere solaire, ont pre'sente les phenomenes de 

 polarisation deja decrits, c'est-a-dire qu'un grand nombre 

 d'entre elles etaient polarisee dans un plan perpendiculaire 

 a leur longueur. 



" Mais en observant avec plus d'attention les moins lumi- 

 neuses de toutes ces lignes, c'est-a-dire celles qui devaient 

 etre les plus fines, on en a trouve un certain nombre qui 

 presentaient un phenomene de sens oppose, c'est-a-dire 

 qu'elles etaient polarisees dans un plan parallele a leur lon- 

 gueur, les unes totalement, les autres partiellement ; cet effet 

 etant accompagne de phenomenes de coloration semblables 

 a ceux qui ont ete signales dans les lignes qui donnent la 

 polarisation perpendiculaire." 



The passage of electric or luminous waves through a fine 

 slit in a thin perfectly conducting screen was considered by 

 me in a memoir published ten years ago*. If the electric 

 vector is parallel to the length of the slit, the amplitude of 

 the transmitted vibration is proportional to the square of the 

 width of the slit ; but if the electric vector is perpendicular 

 to the length of the slit, the transmitted vibration involves 

 the width only as a logarithm — see equation (46) — much as 

 in equation (21) of the present paper. If the incident vibra- 

 tion be unpolarized and the slit be very fine, the latter com- 

 ponent preponderates in the transmitted waves, viz. the 

 direction of polarization is parallel to the length of the slit, 

 in accordance with Fizeau's observations upon light transmitted 

 by apertures of minimum width. 



Terling Place, Witham, July 1907. 



XXX E. Rays of Positive Electricity. 

 By J. J. Thomson, ALA., F.R.SJ 



IN a paper on the rays of positive electricity (Phil. Mag. 

 May 1907) I showed that these rays in gases at very 

 low pressures consisted mainly of streams of two kinds of 

 positively charged particles, the value of e/m for one stream 

 being 10 4 and for the other 5 x 10 3 . As these are respectively 

 the values oiejm for charged atoms and molecules of hydrogen, 

 it might be thought that the rays of positive electricity were 

 dependent on the presence of hydrogen in the discharge-tube. 

 The results described in the previous paper were not, as I 



* Phil. Mag. xliii. p. 259 (1897) ; < Scientific Papers/ iv. p. 283. See 

 also Phil. Mag. July 1907. 



t Communicated by the Author, 



