. The Roentgen rays in medicine and surgery as an aid in diagnosis and as a therapeutic agent : designed for the use of practitioners and students . °, and thus the apparentradiant area is made smaller in its longest diameter and approachesmore nearly to a circular form. Resistance. — For good differentiation between tissues that are notvery different in permeability the tube must have a low for lowering the resistance when it has become too high, and 603 6o4 THE ROENTGEN RAYS IN MEDICINE AND SURGERY obtaining again the proper quality of light, have been stated in ChapterII,
. The Roentgen rays in medicine and surgery as an aid in diagnosis and as a therapeutic agent : designed for the use of practitioners and students . °, and thus the apparentradiant area is made smaller in its longest diameter and approachesmore nearly to a circular form. Resistance. — For good differentiation between tissues that are notvery different in permeability the tube must have a low for lowering the resistance when it has become too high, and 603 6o4 THE ROENTGEN RAYS IN MEDICINE AND SURGERY obtaining again the proper quality of light, have been stated in ChapterII, page 44. The following cut (see Fig. 355) shows the oven therementioned. It has asbestos ends CC, through which pass the terminalsDD. It is heated by the burners HH, the effect of the heat on the tube,which is supported by asbestos slabs AA, being observed through atransradiable door. The heat liberates the. gas from the glass walls ofthe tube. But all methods of lowering the resistance are temporary. Rollins believes that one cause of the rise in the resistance of a tubeis due to a diminished supply of gas particles in the terminals, and. Fig. 355. Oven for heating tube, thereby hberating gas from its walls and lowering its resistance. that it is a necessary result of use. He therefore considers that toreduce this resistance some method should be devised to restore thesegases to the terminals. Tube-holder and Diaphragm. — In addition to a suitable apparatusand tube for producing a small and steady radiant area, it is essentialfor obtaining the best definition to so enclose the tube that no X-raysikn escape except the smallest cone that will cover the area to be photo-graphed. Figure 42, Chapter II, page 52, shows a suitable tube-holderwith cover removed, and Fig. 356 shows the cover and diaphragm inplace. DENTAL SURGERY 605 Screen. — While taking radiographs of the teeth it is, of course,important to avoid danger of burns ; an aluminum screen, grounded, as
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