156 BIOLOGICAL EFFECTS OF RADIATION 



Qualitative data, however, are available for most sources, in most 

 cases photographic spectrograms, where on reduction the intensity is 

 indicated by an arbitrary specification from 1 to 100. In general, the 

 plate sensitivity is either not indicated or is only roughly determined. 

 For certain restricted types of spectroscopic investigation, photographic 

 methods have been highly developed. These methods present even 

 greater complexities than do the radiometric methods. For the pro- 

 cedure of this method, the reader is referred to the work of Harrison 

 (36, 37). 



In the methods outlined above, it has been assumed that a black-body 

 detector has been used in conjunction with a monochromator. Since 

 such black-body detectors of adequate sensitivity require calibration by 

 means of a known source of total radiation, we must be provided with a 

 standard lamp. If a source is available, known as to wave-length 

 distribution of intensity, a somewhat simpler procedure is available, that 

 of differential comparison. The unknown source may be interchanged 

 with the known source, either mechanically or by a suitable optical sys- 

 tem which provides an identical method of illumination. Thus, a black 

 body of known temperature may be interchanged with any continuous 

 source of radiation. Then one obtains simply a factor (aperture, trans- 

 mission, and resolution all canceling out). Or again, an accurately 

 calibrated mercury arc may be interchanged with a mercury arc of 

 unknown intensity distribution. If, however, a selective emitter, such 

 as a mercury arc, be compared with a continuous source, such as a black 

 body, care must be taken to take duly into account the fact that the 

 radiation from the continuous source occupies a wave-length range 

 greater than the resolution of the instrument, while the selective emission 

 of a given line occupies a wave-length range less than the resolution of 

 the instrument. Thus, in the case of the selective source, a change of 

 slit-width only changes the area over which radiation is subtended, the 

 second slit merely obstructing extraneous wave-lengths. In the case 

 of the continuous emitter, the second slit restricts the wave-length band 

 which escapes the monochromator. Furthermore, in calculating the 

 extent of this band, one must take into account the intensity distribution 

 according to wave-length of such transmitted radiation, integration being 

 performed over the wave-length range. For the substitution method, 

 provided that one is using similar types of sources and that the slit width 

 is such that no appreciable change in sensitivity takes place over the 

 range of wave-lengths involved, selective detectors may be used instead 

 of a black body. It is necessary, however, either to assure oneself that 

 the response is linearly proportional to the power received, or if nonlinear, 

 that the detector be calibrated for power as a function of deflection. 



