. Röntgen ray diagnosis and therapy . the fact that the intratubal pressure must be reducedto about two millionths of atmospheric pressure. It is only thenthat the remaining gas becomes radiant. This rarefaction of theintratubal air is brought to this height by suitable exhaust pumps. The cathode rays emanate from the aluminum disk. Theirfocussing point is situated on the platinum of the anticathode. Ifthis point is situated exteriorly or posteriorly from the platinuminstead of being reached directly there are no so-called focus rays,and the tube is practically useless. Another difficulty enco
. Röntgen ray diagnosis and therapy . the fact that the intratubal pressure must be reducedto about two millionths of atmospheric pressure. It is only thenthat the remaining gas becomes radiant. This rarefaction of theintratubal air is brought to this height by suitable exhaust pumps. The cathode rays emanate from the aluminum disk. Theirfocussing point is situated on the platinum of the anticathode. Ifthis point is situated exteriorly or posteriorly from the platinuminstead of being reached directly there are no so-called focus rays,and the tube is practically useless. Another difficulty encountered in the use of the tubes is dueto their soon becoming inefficient on account of the permanentchange of pressure that occurs within them. The cathode raysstriking the platinum disk and the glass walls generate a certainamount of heat. The higher the temperature, the more the amountof current passing through is increased, and the more the vacuumis lowered. Unless the heat is dissipated again at once the tube3 18 THE RONTGEN RAYS. is unstable. On the other hand, as soon as the tube becomescolder the vacuum increases. Consequently the current must over-come a greater resistance, a lesser amount of it passes through, andtherefore a lesser amount of rays is generated. In view of this variation of pressure, tubes have been con-structed that permit lowering and raising of the vacuum withinthem at will. Siemens found that the fluorescing air forms densebodies with the vapours of phosphorus, hydrate of potassium,iodine, and other similar substances, thereby diminishing the press-sure within the tube. On the other hand, if the walls of the tubeare warmed, the stratum of air that condenses on the glass surfaceis driven away, thereby intensifying the pressure. In utilizingthis principle, tubes with adjustable vacuum have been constructed which are providedwith an adjuster,shortening the spacebetween the sparkrods. The self-regulatingtube of Queen & Co.(Philadelphia) is con-structed
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