[cox ><: caluendar] EXPERIMENTS ON THE X-RAYS 173 



to be very expeditious, and it does not appear to be generally known or 

 employed. On first exhausting a tube, the chief difïieulty is generally 

 to get rid of the last traces of gas from the electrodes and the walls of 

 the tube, when high vacua are required for X-ray work. If tliis gas is 

 not thoroughly removed, the vacuum is liable to subsequent tleterioration. 

 The method which we adopted for this purpose consisted in maintaining 

 a continuous discharge through the tube, durina: the process of exhaus- 

 tion, by means of an alternatiyuj current applied to the induction coil, 

 the strength of the current being so regulated as to heat the electrodes of 

 the tube as much as possible without melting them or causing a deposit 

 on the glass. The effect of the current was simultaneously to heat the 

 walls of the tube sufficiently to dry them very completely without an}^ 

 risk of cracking the glass, as may often happen if the tube is artificially 

 heated by means of a Bunsen flame. Starting with a five inch bulb, wet 

 and dirty from the blowpipe, we Avere able in this manner to raise it to 

 an X-ray vacuum in from half an hour to an hour. 



The pump which we generally used for exhausting the tubes, was a 

 five-fall Sprengel of German make, which had been fitted in the labora- 

 tory with a vacuum trap for drying the mercury, and with an automatic 

 arrangement for returning the mercury lo the up])er reservoir. At its 

 maximum rate of working, this pump took only ten or fifteen minutes 

 to raise a five inch bulb from one millimetre to a sparkless vacuum, if the 

 electrodes had been previously freed from gas by the method above 

 described. We found it preferable to the Geissler form of mercury 

 pump, as it permitted the vacuum to be varied continaoudy, and to be 

 maintained at any desired point. We also used, on several occasions, an 

 automatic Geissler pump of the Max Stuhl ])attern. 



Phenomena ijresented by the Focus Tube.- — The phenomena presented 

 by this tube in action, have frequently been described, but the published 

 descriptions do not altogether agree with our experience. According 

 to one account which we received, the kathode rays were regularly' 

 reflected in a small pencil from the platinum plate, and formed a minute 

 focus point on the glass, from which the X-rays proceeded. On exhaust- 

 ing our focus bulb for the fir.st time, we found on the contrary that a 

 whole hemisphere of the glass surface on the side exposed to the kathode 

 rays reflected from the jilatinum, became brilliantly and almost uniformly 

 phosphorescent. We further verified, by taking a ]nn-hole photograph, 

 that practically the whole of the X-radiation came from the focus point 

 on the platinum ^^late, and passed directly through the glass without 

 further diffusion. According to a statement by Professor Lodge, which 

 we observed at a subsequent date, the X-radiation is rendered more 

 brilliant by connecting the platinum plate to the anode, and is diminished 

 in intci sity by allowing the plate to become red hot. We have not been 

 able to observe these effects, and are inclined to attribute them to change 

 in the vacuum, or to tome other peculiarity in the tubes used by Lodge 



