. Journal of radiology . stray radia-tion. Protection of this kind becomesexceedingly important, if high voltagesare used in producing the x-rays. Ad-ditional protection has been obtainedby fastening a lead plate one centimeterthick against the wall on the side to-ward the tube. The target lies in such a position thatthe rays that go through the hole leavethe target in directions almost parallelto the surface of the target. This is ofgreat importance when a detailed analy-sis of the spectrum is desired, for underthese conditions the source of the raysas seen from the hole approximates toa narr
. Journal of radiology . stray radia-tion. Protection of this kind becomesexceedingly important, if high voltagesare used in producing the x-rays. Ad-ditional protection has been obtainedby fastening a lead plate one centimeterthick against the wall on the side to-ward the tube. The target lies in such a position thatthe rays that go through the hole leavethe target in directions almost parallelto the surface of the target. This is ofgreat importance when a detailed analy-sis of the spectrum is desired, for underthese conditions the source of the raysas seen from the hole approximates toa narrow line. The arrangement, how-ever, possesses certain disadvantages,since the cathode rays produce some ofthe x-rays at the bottoms of the littleindentations that appear on the surfaceof the target after the x-ray tube hasbeen thoroughly seasoned. The sidesof these little hollows and the smallprotuberances on the targets surfaceabsorb a very perceptible amount ofthe x-rays. The experiment described Quadrant Electrometer. X-RAY SPECTRA PRODUCED UNDER VARIOUS EXPERIMENTAL CONDITIONS—DUANE in section three furnishes a good ex-ample of this absorption by the targetof its own radiation. After emerging from the hole in thewall the x-rays pass through a thincrystal. In the interior of the crystal (ifit lies in the proper position) some ofthe x-rays are reflected, and deviatefrom their straight line courses in ac-cordance with the fundamental dis-coveries of Laue, Friedrich and Knip-ping. These reflected x-rays pass inturn into the ionization chamber, which,of course, must have been placed in theproper position to receive them. Thequadrant electrometer measures theelectrical current flowing across theionization chamber. For an accurate analysis of spectraslit one should be made narrow, thusdefining a narrow beam of x-rays. Slittwo should be wide enough to admit theentire reflected beam into the ionizationchamber. Secondary x-rays pass off inall directions from the points in thecrysta
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