934. Conditions for Sharpness of Image. To obtain a sharp image in the case of true radio graphy, and in the case of radiography with intensifying screens, it is necessary that the source of the X-rays should be as small as possible, that the tube should be far enough away from the subject, and that the subject should be as close to the film as the thickness of the shutter of the cassette on the one hand, and the thickness of the anti-diffusion grid on the other, permit." In the particular case of working with in tensifying screens, it is essential that the screens should be in perfect contact with the emulsion film over their whole surface ; the base of the cassette and its shutter should be perfectly fiat, and the method of closing the cassette should ensure that the film and the screen are forced into contact.
Under good conditions, a piece of the wire gauze of a No. 40 sieve (40 wires to the inch), placed parallel to, and 41 in. from, the film, with the anti-cathode 31 in. from the film, should give a radiograph in which the mesh should be distinctly visible. Local faults of sharpness indicate bad contact between the screen and the sensitive film ; a general lack of sharpness implies a source of rays which is too large or the existence of secondary sources.
935. Identification of Negatives. X-ray nega tives are generally identified by placing movable lead characters on the cassette during the exposure ; these characters may be made bv sticking a piece of fuse wire between two thin cards or between a piece of plaster and a piece of celluloid, or by spreading lead dust on to an inscription made on thin card by an adhesive mixture such as ink containing gum, or, finally, by pressing lead powder into a film of wax coated on to a card. It is also possible to impress a sheet of lead with all required particulars, e.g. series number, date, etc., by using a cheque perforating machine.
By taking care that the inscriptions are always placed in the same position relative to the subject, they may be used to show the correct placing of the negative when it is examined.
936. Time of Exposure. The optimum time of exposure depends in radiography, as in ordi nary photography, on many factors, and it is necessary to understand the influence of these factors.
Other things being equal, the optimum time of exposure is proportional to the square of the distance of the anti-cathode from the sensitive film ; for example, in moving from 6o to 8o cm., the time of exposure would be increased in the proportion 36 to 64, i.e. I to 1-8.
The optimum time of exposure is inversely proportional to the intensity of the current which flows through the generator. When
changing, for example, from an intensity of 10 rnilli-amperes to one of 15, the time of ex posure would be reduced in the proportion of 1-5 to I.O.
In the absence of an intensifying screen, the time of exposure is almost inversely proportional to the square of the difference of potential. For example, in passing from 55 to 70 kilovolts, the time of exposure would be reduced in the ratio of approximately 5 to 3. In the case of using intensifying screens no rule can be formu lated, since it is known that the effectiveness of screens varies considerably with the quality of the radiation emitted, and consequently with the voltage and form of the current and also with the method of preparation of the screen used.
The time of exposure varies almost propor tionately with the square of the thickness of the subject, it being understood that the comparison is made between bodies of the same nature.
The time of exposure varies with the sensi tivity of the emulsion used, but it should be remembered that there is generally no pro portionality between the sensitivities of two emul sions towards X-rays (radiography, strictly speak ing) and towards fluorescent light (radiography with intensifying screens) ; finally, it varies slightly with conditions of development (§ 937).
In every case systematic tests should be made to determine the optimum time of exposure, which depends in large measure on the equip ment used (source of electricity and the X-ray generator) ; in these tests no other factor than the time of exposure should be allowed to vary. Once the optimum exposure for determined conditions has been found, the time for other experimental conditions can be calculated by application of the rules given above.
Numerous calculators of exposure for radio graphy give an approximate figure for the time of exposure under certain definite conditions by means of graduated sliding scales, some of which are worked out from theoretical considerations and others from experimental determinations.
For timing the very short exposures often. rendered necessary when using powerful ap paratus, seconds-counters are frequently used. These are graduated in seconds or in milli ampere-seconds (product of the intensity and the time), thus providing automatically for accidental change in the intensity.' 937. Photographic Operations. The purely pho tographic side of radiographic manipulations is often neglected, with the risk that a large part of the advantages accruing from a good installa tion and from highly-sensitive films may be lost.