X-RAY
CHAPTER
X-RAY
VII
Screens - Lippmann
129
are needed for penetration of deep or thick parts of the body or for metallographic work, the shorter or u soft" rays produce the greatest blackening. For study of the sensitornetry of X-ray photography the reader is referred to the work of Wilsey and
A ID ULTRAVIOLET
X-ray Emulsions - Intensifying tra violet Plates.
A D ULTRAVIOLET
Emulsions - Ul-
Pritchard." In medical practice, the peak voltage used, which is a direct measure of the penetrative power, varies between 35 and 90 .40
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fir~t few years following. ~oentgen'~ discovery of the X-rays, It was found by empirical expenrnent that plates coated with a slow silver bromide emulsion containing the least po.ssible quantit~. of s~lver iodide, gave the fastest response. HIgh speed to visible light had very little to do with the matter. It must be remembered that at this time X-rays were produced with very crude tubes, excited by Ruhrnkorff coils ~tt~d with the old hammer type of interrupter, capable of givmg a spark in air of only a few inches. Exposures of as much as thirty minutes were quite common for thick parts of the body, and it was not at first understood that short-wave radiation had a greater penetrative power, without which the longer waves could not produce a result, no matter what quan.tity of energy was used. Fig. 35 shows how the penetrative power increases with decrease in wavelength in the case of aluminum 1 While the action of X-rays on photographic emulsion films is very similar to that of visible light, examination of microscopic sections through the developed film shows that the rays produce an equal distribution of grains of reduced silver thro~ghout its whole thickness. It is thus evident that greater denslt~ ~an ~e obtained by increasing the thickness of the layer of sensitive SIlver salts, or alternatively its silver halide content. As stated, however, X-rays of different frequencies do not have an equal quantitative effect, and while the shorter wavelengths
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kilovolts. This is ordinarily obtained from a transformer, and after rectification is applied to the tube. The modern X-ray tube has a cathode which is heated by a local circuit, and as the heat is increased, so the electrons emitted by it lower the resistance of the path between it and the anode, and so control the hardness of the radiation generated at the anode surface. Over this range the one type of X-ray emulsion functions so satisfactorily that different emulsions have never been made for work in different parts of the X-ray spectrum. According to Charlesby," the response of a photographic emulsion to incident X-radiation is appreciably affected by the