1896.] on Electric Shadows and Luminescence. 195 



tliis subject of electric discharges in the vacuum tube. But to under- 

 stand the discoveries of Crookes let us first witness a few experimental 

 illustrations of the phenomena of electric discharges in vacuum tubes. 

 Many of them have been known for half a century. We all know of 

 the researches made in England by Gassiot, and by Varley and others, 

 and the tubes of Geissler of Bonn are a household word. But there 

 is one set of researches which deserves to be known far better than it 

 is, that made by Dr. W. H. Th. Meyer, of Frankfort, whose pamphlet * 

 I hold in my hand. In it he depicts a number of tubes in various 

 stages of exhaustion, including one in that highest stage of exhaustion 

 which one is j^rone to think of modern origin. 



In order to illustrate the successive phenomena which are pro- 

 duced when electric discharges are sent through a tube during 

 progressively increasing exhaustion, there is here exhibited a set of 

 identical tubes. Each is a simple straight tube, having sealed in 

 at each end an electrode terminating in a short piece of aluminium 

 wire. The electrode by which the electric current enters is known 

 as the anode, that by which it leaves the tube as the kathode. Tlie 

 only ditference between these eight tubes lies in the degree of rare- 

 faction of the interior air. The first one contains air at the ordinary 

 pressure. As its electrodes are about 12 inches apart I am unable 

 with the Apps induction coil (excited to throw an 8 -inch spark) to 

 send a spark through it. From the second tube about four-tifths of 

 the air has been abstracted, and here we obtain a forked brush-like 

 spark between the electrodes. The third tube has been exhausted to 

 about one-twentieth part, and shows as the discharge a single thin red 

 linear spark like a flexible luminous thread. When, as in the fourth 

 tube, the exhaustion is carried so far as to leave but one-fortieth, the 

 red line is found to have widened out into a luminous band which 

 extends from pole to j)ole, while a violet mantle makes its appearance 

 at each end and spreads over both of the electrodes. On carrying 

 the exhaustion to the stage shown by the fifth tube, where only about 

 gi^ of the original air is left behind, we note that the luminous 

 column has broken ujd transversely into flickering strise, that the violet 

 mantle round the kathode has become more distinct, and is separated 

 by a dark interval from the luminous red column, while a second and 

 very narrow dark space appears to separate the violet mantle from 

 the surface of the kathode. In the sixth tube the exhaustion has been 

 carried to about xo^oo- "^^^ flickering strise have changed shape and 

 colour, being paler. The light at the anode has dwindled to a small 

 bright patch. The violet glow surrounding the kathode has expanded 

 to till the whole of that end of the tube ; the dark space has become 

 more distinct, and within it the kathode now shows on its surface an 

 inner mantle of dull red light. There is a slight tendency for the 



* Beobachtuntjen iiber das gescliichtete electrische Licht, sowie iiber den 

 merkwiirdigeu Eintinss dcs Magueten auf dasselbe ; von Dr. W. H. Theodor 

 Mever. Berlin, 1^5^. 



o 2 



