. Dental electro-therapeutics. The Lambertz stand. THE X-RAYS OR RONTGEN RAYS 171 turned on and the rays allowed to pass through the tube, asmall and perfect luminous disk should appear on the fluor-escent screen. The cylinder of x-ray-proof material, providedwith an adjustable diagram, cuts out the secondary rayswhich do so much to impair the sharpness of the radio-graph, and of which more will be said later. When the operator is in a position to direct the rays in astraight line through the middle of the cylinder, the nextquestion he must consider is the relation of this line; firstly,to the
. Dental electro-therapeutics. The Lambertz stand. THE X-RAYS OR RONTGEN RAYS 171 turned on and the rays allowed to pass through the tube, asmall and perfect luminous disk should appear on the fluor-escent screen. The cylinder of x-ray-proof material, providedwith an adjustable diagram, cuts out the secondary rayswhich do so much to impair the sharpness of the radio-graph, and of which more will be said later. When the operator is in a position to direct the rays in astraight line through the middle of the cylinder, the nextquestion he must consider is the relation of this line; firstly,to the axis of the tooth or root, and, secondly, to the planeof the film or plate; and here he must observe certain prin-ciples of projection which in themselves are simple, butwhich it may be well to recall. Fig. 76 The first is the consideration of the relative distancesbetween the target, the object to be x-rayed and the filmor plate, and the effect of these upon the size of the skia-graph compared to the object itself. This may be easilyillustrated by the accompanying diagrams, and roughlystated by the following self-evident rule. T being the target, cd and Cidi the object to be x-rayed,and p the film or plate, the nearer the object cd is to the 172 ELECTRO-PHYSIC* plate, the smaller the image ab will be, and the furtherfrom the plate, the larger the image cdi will be on plateaidi; the distance from T to the plate remaining the same;and as shown in Fig. 77 the distance between the objectcd and plate p, remaining the same, the image of the objectwill be larger, the nearer the target is to the plate, abproduced by the rays starting at T is longer than afii withthe target at J\. The distance between the target and the plate or filmin dental radiographs should usually be about 35 cm. withthe film
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