462 J O H N W. G O W E N 



while filtering out much of the general radiation on either side of this 

 peak. Consequently, the radiation after this filtration is more nearly 

 monochromatic. The actual choice of a filter depends largely on its 

 atomic number. In general, the element of choice is one or two 

 atomic numbers below the metal used as the anode of the X-ray tube. 

 Palladium is used to filter X rays from silver anode tubes, zirconium 

 for molybdenum, nickel for copper, manganese for iron, and vanadium 

 for chromium. The short wavelength limit of this method of filtra- 

 tion occurs at about 0.1 A.; it is applicable, then, only to X rays of 

 medium or long wavelength. Graphical representation of the effects 

 is shown in Figure 4. 



A further difficulty in obtaining monochromatic X rays is the 

 alternating potential ordinarily used to operate the tubes. The al- 

 ternating rise and fall of the high voltage supply (c/. 5, Fig. 57) re- 

 sults in a consequent shortening and lengthening of the wavelengths 

 of the X rays produced. Various schemes are used to reduce this ef- 

 fect, but some voltage ripple is almost always present. If adequate 

 rectification is not available to eliminate this effect, then, at best, 

 filter techniques will lead to a rather broad band of wavelengths. 

 Direct current sources could be used, of course, especially for low 

 voltage studies. 



For some biological investigations, the Ross {4, P- 532) double 

 filter technique might prove useful. It is based on differential ab- 

 sorption by metallic filters. 



C. THEORIES OF X-RAY ACTIONS 



1. The ''Point-Heat" Concept 



Historically, the first general theoiy designed to explain the re- 

 sults observed in the treatment of tissues by X rays was that of Des- 

 sauer {15). It was recognized that radiant energy, in passing through 

 gases or tissues, was subdivided into discrete units as it underwent ab- 

 sorption in these substances. A particulate or quantum theory of 

 X-ray absorption was evolved. At the point of release of the energy 

 of these quanta, two types of effects were conceived. The first 

 looked on the absorbed energy as generating a considerable amount 

 of heat at the small focus of the release. This theory was developed 

 into the "point-heat" theory of X-ray effects by Dessauer. The 

 theory accounted for the very specific and localized effects of the X 



