484 
DR. S. P. THOMPSON ON CATHODE RAYS 
on the operation of the deflecting magnet in separating the different elements of a 
bundle emitted simultaneously at a cathode. The remarkable success of Lexaed in 
transmitting cathode rays through a window of aluminium leaf into the open air 
naturally suggested the possibility of partially separating within the tube itself the 
constituents of a heterogeneous cathode beam, by interposing screens or films to sift 
out the less penetrating from the more penetrating parts. 
Accordingly, a tube [No. G 19j, (fig. 17), was prepared, having at one end a small 
disk cathode, K, of aluminium, and an aluminium wme anode, A, in a bulb in the 
lateral exhausting tube. An interior luminescible screen, S, of mica coated with 
scheelite was inserted near the far end of the tube, and in front of this was erected, 
upon a conducting support, as an object to cast shadows, a cross, one arm of which 
was of aluminium foil, another of platinum foil, whilst the third was of platinum 
upon which a globule of glass was fused. Near the cathode the tube was fashioned 
with a shoulder, into which loosely dropped a ring of lead, carrying a diaphragm of 
aluminium foil. A jfiatinum wire, C, inserted through the glass, enabled this ring 
and diaphragm, when in place, to be connected up to anode or cathode if desired. 
The ring and diaphragm being removable by tilting the tube, ordinary cathodic 
shadows could be produced for comparison. The tube was exhausted in the usual 
way to a high degree, and sealed ofi’ the pump. When the diaphragm was shaken 
out of its seat to allow a clear passage of the cathode rays, the shadows of the object 
upon the screen, S, were very sharp and distinct, and all parts of the shadow were 
equally dark. When the diaphragm was replaced, there was a shadow also, provided 
either B or S was earthed, or connected to the anode through a resistance, or made 
itself the anode. If C (the diaphragm) was connected to the cathode, the shadow 
became more brilliant, but with so rapid volatilization of lead, or of occluded gases, 
that this connexion was only possible for a second or two. The shadow was in all 
cases deflectible by the magnet, and appeared to be more readily deflected if the 
magnet was applied over the region between the diaphragm and the object than if 
applied between the cathode and the diaphragm. Althoufifh the diaphragm fitted 
fairly well in its seat, which was lined with aluminium foil, and permitted no direct 
cathode discharge around its edges, there was a distinct luminescence of the glass at 
the far end of the bulb around the edges of the mica screen, which cast a well- 
defined, dim shadow. The colour of this luminescence was the usual yellow-green, 
but there was a singular dark-orange luminescence over the end of the bulb where 
screened by the mica disk. When the aluminium diaphragm was connected to the 
cathode, and the platinum support of the mica screen was connected to the anode, a 
similar orange luminescence appeared in the neighbourhood of the cathode, K, during 
the brief moments that the operation could be continued. Further reference is made 
below to this second species of luminescence. 
* Op. cit. This dis23ersion Tvas also obsei’ved by the author in May, 1S96, see ‘ Pj’OC, Oxford 
University Junior Scientific Club,’ May 26, 1896. 
