. Dental electro-therapeutics. asilyillustrated 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 th
. Dental electro-therapeutics. asilyillustrated 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 pressed as closely to the alveolus as possible. Intaking radiographs of the teeth of the upper jaw; with theplate or film held between the teeth and taken from above,the distance should not be less than 35 to 40 cm. Fig. 78. The direction of the rays in their relation to the axis ofthe roots (especially in the upper teeth) and the plane ofthe film is of great importance in producing an approxi-mately correct image of the tooth or root on the film, andhere the rules advocated by Cieszynski and Dieck shouldbe observed as closely as possible, provided there are noother conditions, such as can only be brought out by adistorted picture; this, however, will be referred to later. THE X-RAYS OR RONTGEN RAYS 173 The rule to be observed is to direct the rays verticallythrough the apex of the root in question on to an imaginaryline dividing the angle formed by the axis of the root andthe plane of the film. This is shown in Fig. 78. An instrument devised by Dieck for this purpose isshown in Fig. 79 Fig. 79
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