. The book of photography; practical, theoretical and applied. ic current istraversing it is due to the fluorescenceset up in the glass by the bombardmentof the particles which remain within thevessel. The condition of the tube may besafely gauged by the evenness with whichthis green glow seems to fill the tube, andit is generally found that if this is at itsbest, the most satisfactory photographiceffects may be expected. Dr. HallEdwards, an experienced worker with the X-rays, says that he can always tellhow the tubes are working by noting thevisible amount of fluorescence. Anotherwell-known e
. The book of photography; practical, theoretical and applied. ic current istraversing it is due to the fluorescenceset up in the glass by the bombardmentof the particles which remain within thevessel. The condition of the tube may besafely gauged by the evenness with whichthis green glow seems to fill the tube, andit is generally found that if this is at itsbest, the most satisfactory photographiceffects may be expected. Dr. HallEdwards, an experienced worker with the X-rays, says that he can always tellhow the tubes are working by noting thevisible amount of fluorescence. Anotherwell-known experimenter, Mr. Isenthal,however, seems to be of the contraryopinion, having stated that the fluores-cence of the tube cannot be taken as acriterion of the amount of exposure to begiven when using the tube with a photo-graphic plate. He asserts that he hasused tubes which fluoresce excellently, andwith which he can discern upon a screenmany of the deeply seated structures ofthe body, and that those same tubestotally fail to afford satisfactory Fig. Jiy.—AlTAltATrs FOi; rjionrcTiox OFX-K.\YS. Exhaustion of the Tube, The X-ray tube is exhausted of its con-tained air by the Sprengel, or somesimilar form of mercury pump, and theexhaustion is generally carried to aboutone-millionth of an atmosphere. This isa high vacuum, but very far from thetheoretical perfect vacuum. Sir WilliamCrookes has pointed out that it is only ahigh vacuum by courtesy, and he showsthat, although it may seem that whenthe air is i-educed to one-millionth part ofits bulk the residue must be trifling, thisidea is fallacious. A bulb of glass fiveinches in diameter contains more than aquadrillion molecules, and when it is ex-hausted to one millionth of an atmospliereit will still contain more than a trillion. 708 THE BOOK OF PHOTOGEAPHY. So that an X-ray tube with a five-inch bulb, having what we call ahigh vacuum, .contains more than1,000,000,000,000,000,000 molecules of air. Using Current from
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