The dark-room should contain a cupboard, lined inside with tin. lead, to hold charged cassettes and sensitive material in use. A similar cup board should be provided, preferably outside the dark-room, for stocks of plates, films, and papers.
Every precaution should be taken to avoid bruising or bending, however lightly, radio graphic films, the emulsion of which is very susceptible to abrasion (§ 199).
Desensitizing is of particular advantage in radiography in that, as it decreases the risk of fog, it allows development to be prolonged until the desired degree of contrast has been obtained.
Development is best done in vertical tanks, using developing frames (§ 262) as radiographic negatives always have heavy fog, caused by the diffused rays, and which is very intense if an anti-diffusing grid is not used, the contrast reaches a maximum and then falls off very rapidly if development is forced. An endeavour should be made to develop to maximum con trast and thus to assure the best visibility of details in regions where the permeability to radiation is small. Now the range of develop ment times giving the desired result is much wider with a bromided developer than with a. non-bromided or insufficiently bromided devel oper. Owing to the regression of inertia (§ 337), and to the passage of the contrast through a maximum, it is possible to obtain within this range a satisfactory balance between the tune of exposure and the time of development analogous to that already noted in the case of gelatino-bromide papers (§ 557). By forcing development to this relative degree, a reduction in the exposure of nearly one-half may be made (R. B. Wilsey, 1925).
In a developer such as Nieto]. (Elon, Viterol, etc.) . 22 gr. (2.5 grin.) Sodium sulphite, anhydrous 2 OZ. (too grin.) I rydroqu n one• . . go gr. (to grm.) Sodium carbonate, anhydrous . i oz. (50 grin.) Potassium bromide . . 18 gr. (2 grm.) Water, to make . 20 OZ_ (1000 C.C.) it is best to adjust the duration of development to twelve times the time taken for the appear ance of the image (§ 34.4.). As, moreover, the supervision of the development of each exposure separately removes all the advantages of the use of vertical baths, it is best to determine the normal time of development every now and then (to allow for the progressive exhaustion of the bath and its variations in temperature), and to develop the negatives automatically for the time thus ascertained.
This determination of the normal time of development can be made very simply and quickly as follows (R. W. Wilsey, 1925) : Fog half the width of a strip of film by covering the other half with a thick strip of lead. and exposing the whole to X-rays without an intensifying screen (15 milli-ampere seconds at 6o kilovolts at 28 in. from the anti-cathode), and cut the film transversely into several pieces so that each piece has a fogged and an unfogged portion ; these pieces are kept for use as needed. The total optimum time of development, equal to 12 times the time taken for the appearance of fog on the half of the sample, is determined by developing the test film, without previous desensitizing, in a small dish into which has been placed the necessary quantity of developer from the tank. As this test takes scarcely more than a minute, it can be repeated so that the mean of the two results can be used to give greater exactness. The time thus found is multiplied by 12.
The same procedure is followed for other developers after having found experimentally the optimum value of the Watkins " factor." All other operations are carried out in accord ance with the general recommendations already given for photographic negatives.
The negative in this case being usually the final image and not an intermediate one as in photography, its essential quality is legibility in the normal conditions of examination. This rules out densities appreciably higher than 2, unless the lamps used for examination are increased in intensity. Now, in films coated on both sides; with a layer of emulsion, normally exposed and developed to maximum contrast, the straight portion of the density curve only begins at density 2. The negative that the radiologist considers as correctly exposed is, therefore, usually an under-exposed negative, in which the contrast is much less than it could be if all the densities were included in the straight portion of the characteristic curve. In the case of subjects of weak contrast, it would be advantageous to expose more fully, subject to applying superficial reduction, to bring back the negative to a condition in which it is legible (M. Abribat and J. Thoumas, 1928).