Protection against X-rays as such.
We will summarize here the recommendations made in 1925 by the National Physical Labora tory (London)— The generating tube should be enclosed as completely as possible in a suitable protecting material (cup), provided with only one aperture, and this as small as possible. This container should be equivalent to at least 2 MM. in.) of lead (x-5 mm. if the installation does not exceed 70 kilovolts). The diaphragm, which limits the field irradiated, should be equivalent to 3 mm. of lead (2 mm. for less than 70 kilo volts), and should be made to close completely.' A screen 43 in. wide, 7 ft. high, and reaching to within x in. of the floor, should be in front of the operator ; it should be equivalent in thick ness to at least 2 mm. of lead. If the screen is fitted with a glass window (special lead glass), its protective value should not be less than 2 mm. of lead, nor should it be larger than 22 X 15 cm. (9 X 6 in.).
If the voltage is greater than zoo kilovolts, the protection should correspond with at least 3 mm. of lead ; the walls of the room, and, if necessary, the floor and the ceiling, should be provided with protection equivalent to at least 3 mm. of lead.
The thick metal rods terminated by spheres should, as far as possible, be replaced by wire conductors. Overhead should be at least xi ft. from the floor. It is recommended that the nearest portions should be protected by insulating tubes with thick walls. All metal portions of the apparatus should be earthed.
The control levers should be readily accessible and distinctly labelled. The switches and cut outs should be double-pole, and the fuses should not be of excessive capacity.
It is especially recommended that the X-ray laboratory should not be installed in a basement ; the height of the ceiling should not be less than 12/ ft. ; damp places should be avoided. Effec tive ventilation should be provided to ensure elimination of ozone and nitrous vapours.
Operators should not work for more than 7 hours per day ; there should be one day and two half-days free each week and a month's holiday each year, preferably in the form of two separate fortnights.
To the above may be added the caution that if the photographers and the assistants attached to the laboratory do not use the same means of protection as are used by medical radiologists (aprons and gloves impregnated with lead salts, and protective spectacles), they should avoid getting within reach of direct or diffused radiation.
928. Radiographic Images. X-ray images may be compared with shadows projected by a source of light. As is well-known, shadows are sharper as the source of light approaches a point, in which case the shadow obtained approximates to a perspective view obtained from the source of emission.' In the case of radiography, the source of emission is never a point (diameter about in.), and, therefore, the generator should be removed to such a distance that the source, when viewed from the subject, approximates to a The general laws of perspective (§ 22-29) are applicable to the formation of radiographic images. In particular, if it is desired to avoid a marked disproportion in the scale on which are reproduced planes which are at various distances from the plane of projection, the principal distance should be greater as the object to be radiographed is thicker, e.g. at least 3 ft. for the thick portions of the body of an adult subject of medium size.
929. Photographic Effects of X-rays. A sensi tive emulsion, when exposed to X-rays, absorbs only a very small fraction (less than r per cent) of the incident radiation ; the latent image, instead of being concentrated in the upper surface of the emulsion, as it is in the case of exposure to light, is uniformly disseminated throughout the whole thickness of the emulsion.' Thus, several almost identical images may be obtained by superimposing a series of sensitive emulsion coatings, provided that the supports are of slight absorption (film or paper).