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

356 THE ADVANCE OF PHOTOGRAPHY without any danger of the insulation breaking down. The current is made and broken under oil by an automatic timing switch, so that it is possible to time the exposure in hundredths of a second. In the figure, the stand to the left shows an arrangement in which, by means of a trigger, the operator causes a weight to fall, and this quickly makes and breaks the current by the aid of the oil switches. In the negative obtained, the more dense parts, e.g. thebones in the hand, will appear white on a dark background. The amount of exposure required is governed by many circumstances, such as the nature of the subject, the hardness of the tube, the size of the coil, the primary current, and the distance of the tube from the plate. Hard tubes require shorter exposures than soft tubes, but negatives obtained by hard tubes are very apt to be thin. To radiograph the bones of the hand, Messrs Cox recommend an exposure of five to ten seconds, when using a 10-inch coil and the ordinary break, while with the same coil, twenty to thirty seconds exposure is necessary to obtain a successful negative of the ankle bones. The thorax under the same conditions requires still longer, and the pelvis must be exposed for about one minute. When a larger current is sent through the tube, by using an electrolytic break in place of the hammer break, these times are of course in all cases considerably reduced. The nature of the photographs obtained with a tube depends to a very great extent on the hardness of the tube. This is very clearly shown in the three figures (159, 160,, 160a). The first of these is taken with a soft tube, that is onein which the vacuum is not a sufficiently high one, and it can be at once seen that even the less dense parts throw quite a distinct shadow, and consequently there is a lack of good definition. The second shows what definition and distinctness can