486 
DR. S. P. THOMPSON ON CATHODE RAYS 
to be described only the thinnest of these filters was used ; it was a leaf about 
0'07 millim. thick. The leaden slip was itself connected by a thin spiral of platinum 
wire to the wire leading out through the stopper. Into the lower horizontal tube 
was introduced a similar loose slip of sheet-lead, carrying upon its upper surface a 
number of fluorescible materials : scheelite, ruby, jacinth, diamond, Iceland-spar, fluor¬ 
spar, calcium sulphide, zinc sulphide (Sidot’s blende), and a preparation of calcium 
carbonate, containing in solid solution about per cent, of manganese carbonate. 
The disposition of the two horizontal tubes at right angles to one another enabled the 
two lead carriers to be moved, by' tapping, Independently of one another, while the 
Fig. 18. 
tube was connected to the pump. This tube was used for sixteen consecutive days 
upon the pump. It was several times pumped out to the point at which Eontgen" 
rays are emitted. When left to itself, the vacuum slowly deteriorated, the tube 
relapsing in two days to the stage below that at which the yellow-green luminescence 
appears. Various observations were made on the luminescibility of the materials 
enumerated above ; but these had little direct bearing on the point now in question. 
When K 2 was used to produce ortho-cathodic ra 3 ^s, all the materials named luminesced 
brightly, most of them doing so even wdren the state of exhaustion was too low for 
the production of the yellow-green luminescence of the glass. They all, also. 
