. Radiography and the 'X' rays in practice and theory : with constructional manipulatory details . ruse, the resulting calcium sulphide must be finelypowdered in a glass mortar and passed through a finelawn sieve. § 53. Having now a stock of the materials we canpass to the preparation of the screen itself. Screensmay be divided into tluee classes: first, those intendedfor direct vision only; secondly, those which are speciallyintended for use in conjunction with photographic platesin the production of radiographs ; and, thirdly, thosewhich serve fairly well for both purposes. To the first clas
. Radiography and the 'X' rays in practice and theory : with constructional manipulatory details . ruse, the resulting calcium sulphide must be finelypowdered in a glass mortar and passed through a finelawn sieve. § 53. Having now a stock of the materials we canpass to the preparation of the screen itself. Screensmay be divided into tluee classes: first, those intendedfor direct vision only; secondly, those which are speciallyintended for use in conjunction with photographic platesin the production of radiographs ; and, thirdly, thosewhich serve fairly well for both purposes. To the first class belong screens made with barium FLUORESCENT SCREENS m platinocyanide, with calcium tungstate (or calciumtungBtate and mica), and with uranium fluoride. Ofthese, there is no douht that the harium jjlatinocyanideis the hest, although good samples of calcium tungstaterun it very close. Uranium fluoride (also known asDr. Melckehekes salt) is good, but decidedly inferior tothe calcium tungstate. For the preparation of screens of this class theoperator should procure a sheet of good white Bristol. Fig. 30.—Spreading Gum-Water. board. This should be thick enough to be ahsolatelyopaque to ordinary light, and should be tested to thisend by holding up in strong sunlight, with the hand orother opaque body behind. A very convenient size forsurgical work is 12 in. by 9 in., but a screen 9 in. by 6 be found large enough for general purposes. Thesheet having been cut to the desired size is pinned on aperfectly flat drawing-board by its four corners. If cal-cium tungstate is to be used, a sufficient quantity should 112 rx.\Djnr,n.\pny be placed close at hand witli a line wire <;aiize Kiove. Asmall pool (about as lari;e as a crown piece) of ^ood ^m\\-water should now bo poured on the centre of the sheetof cardboartl and (juickly spread over its surface bymeans of a pilass rod or tube, bent twice at rij^bt anp(les,as shown at lij;. JiO. Any excess of j^um-water shouldnow be swept
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