190 



BIOLOGICAL EFFECTS OF RADIATION 



Table 17 



Two other types of sources deserve mention. First, where gas dis- 

 charge is maintained under a high pressure, atmospheric or greater, an 

 intense practically continuous emission is obtained in the blue and ultra- 

 violet. This is often the only available source of continuous radiation 

 in that region. The other type of source is the ring discharge, where 

 the excitation is produced by high-frequency induction. These sources 

 are free from difficulties due to electrode deterioration, but since the 

 excitation generally corresponds to very high potentials, an entirely 

 different spectrum from the ordinary arc spectrum is obtained. Some 

 of these sources are now manufactured for aviation-field illumination. 

 They are the most efficient illuminants known. 



By a combination of these sources of selective emission with suitable 

 monochromatic filters, lines can often be isolated, thus yielding a very 

 satisfactory monochromatic source of irradiation. Table 11a shows 

 various combinations which are possible with the mercury arc. Table 

 11b shows corresponding possibilities with helium. Table lie has been 

 worked out for hydrogen. Assuming an arbitrary intensity 100 for the 

 red line, the intensities given would roughly correspond for other lines 

 available. It must be particularly emphasized that the relative inten- 

 sities of lines from such sources vary over a tremendous range with 

 differing conditions of excitation. The characteristics are influenced by 

 temperature, pressure, voltage, and current. Furthermore, the shape 

 and size of the tube are of great importance, as lines are reabsorbed within 

 the source in varying degree. Figures presented are intended to serve 

 only as a basis for selection of equipment, not as dependable operating 

 characteristics. 



SELECTIVE DETECTORS 



In a wide range of problems, one wishes to evaluate energy available 

 in restricted ranges of wave-lengths without resorting to complete 

 monochromatic analysis. This is the case where it is not feasible to 

 employ a monochromator, either on account of the mechanical arrange- 



