1896.] 



on Electric Shadows and Luminescence. 



199 



more or less directly kafchodfc kind in places where they could not 

 have penetrated by any motion in a straight line. 



Here (Fig. 1) is a tube — a variation on one of Hittorf s, having 

 two branches that cross one another at right angles. There are two 

 small disks of aluminium in the bulbous ends to serve as electrodes. 

 When either of these is made the kathode, the whole limb in which it 

 is situated fluoresces brilliantly of a golden-green tint, particularly at 

 the distant end. But the other limb remains dark, save for a little 

 nebulous blue patch, near the anode, due to residual gas. Another 

 tube (Fig. 2) is made as a zigzag, and here again only the end 

 branch shines. On reversing the current the luminescence shifts to 

 the other end. But when the tube is more highly exhausted, the 

 phosphorescence is observed not only in the end branch where the 

 kathode is, but also slightly at the end wall of each branch of the 

 zigzag. Apparently the residual gas will act partly as its own 

 kathode, and throw off something which causes the glass beyond to 

 phosphoresce. 



And now let me remark that not one of all the tubes shown since 

 the first one, is capable of showing a shadow upon the fluorescent 



Fig. 2. 



screen outside, or of taking a photograph through a sheet of aluminium. 

 Even the brilliant tube which showed so excellently the shadow of the 

 cross, fails to show any result after hours of vain waiting. It yields 

 no rays that will penetrate aluminium. For experiments with 

 Roentgen rays it is absolutely necessary that the process of exhaustion 

 should be carried beyond the stage that suffices for the production of 

 kathode shadows ; it must be pushed to about that limit which Crookes 

 himself described as his unit for the degree of vacuum, namely, one- 

 millionth of an atmosphere. I do not say that with long exposures 

 photographs cannot be taken when the degree of exhaustion is lower. 

 Something depends, too, upon the degree to which the electric dis- 

 charge is stimulated, and something also depends upon the shape and 

 structure of the tube and upon the size and shape of the kathode. 

 But on none of these things does the emission of X-rays depend so 

 much as upon the degree of vacuum. The highly exhausted vacuum 

 is the one real essential. 



So soon as Crookes's researches upon electric shadows had become 

 known, electricians set to work to try to produce electric shadows in 

 ordinary air without any vacuum. One of the ablest of exjierimenters, 



