The advance of photography : its history and modern applications (1911)

358 THE ADVANCE OF PHOTOGRAPHY One very important use of radiography is the localisation of foreign bodies, such as bullets, in the human body* Appearances in this respect are very deceptive. If one photograph only is taken it is next to impossible, owing to the great penetrating power of the rays, to ascertain the exact position of the object ; in fact, it is sometimes difficult to even roughly fix its position with reference to the bones. To do this, stereoscopic views are taken, and pieces of apparatus, such as the MackenzieDavidson localisers, have been invented in order to perform this operation in a proper scientific manner. Reversal by Rbntg en-rays. — Rontgen-rays can produce the phenomena of solarisation or reversal of the image on a photographic plate in the same manner as an over exposure of the plate on the camera to very bright objects such as the sun. Experiments on this cause of reversal have been carried out by M. Chanoz.1 He partly covered a sensitive plate with a steel watch-spring, 0*12 mm. thick, the whole being wrapped in black paper. The plate is exposed to the action of the rays, an arrangement being made by means of a lead screen so that strips of the plate could be exposed for varying lengths of time. When the exposed plate is developed, it is found that the density obtained is by no means proportional to the length of the exposure. In the strips which are exposed for under two minutes,, the parts under the spring are much more transparent than the rest. With further exposure the density of the strip gradually becomes more uniform, and by the time the exposure has reached ten minutes, the part under the spring is the denser. By an increase in the length of exposure beyond this point, the density once more becomes uniform, and when the plate has been under the action of the rays for aperiod of between 1J and 2 hours, the part under the steel 1 Comptes Rendus, 146, pp. 172-174, Jan. 27, 1908.