198 BIOLOGICAL EFFECTS OF RADIATION 



obtained both under glass Tg and under quartz T^. Since the effective 

 intensity is inversely proportional to the time of darkening, 



Intensity = If y) ^ ^ 



where C is a calibration factor. By the use of a mercury arc, Clark 

 arrived at a calibration factor, 



C = 72 X 10^ erg/cm.2 



The relative wave-length sensitivity of this differential zinc sulfide 

 method is given by curve ZnS-2 in the center section. While this method 

 is more restricted as to wave-length range than Coblentz's differential 

 thermocouple method, it is nonlinear over the whole region, showing a 

 minimum in the region of 2650 A and a maximum at 3130 A, falling 

 rapidly between 3200 and 3300 A, and is practically zero at 3500 A. 

 This selectivity might be a definite advantage in case the biological 

 effectiveness of dosage were to closely follow the sensitivity distribution. 

 Otherwise, it is rather difficult to evaluate its significance. Measure- 

 ments by this method have been specified in terms of zinc sulfide units. 

 This procedure is good, inasmuch as both wave-length distribution and 

 magnitude are implied. But without a standardization as to glass filter 

 and an accurate determination of the sensitivity distribution of a stand- 

 ard method of procedure, it is difficult to convert these observations into 

 other units. Thus, the curves previously shown for daily and seasonal 

 fluctuation in solar ultra-violet give an excellent idea of the fluctuations, 

 but the wave-length range represented is in a large measure indeterminate, 

 following roughly the curve shown in the lower section of Fig. 30, marked 

 ZnS-2. The maximum response for the particular solar distribution 

 assumed [taken from Forsythe and Christison's observations (28)] occurs 

 at 3200 A and drops to zero roughly at 3600 A. When, however, the 

 short-wave cut-off of solar distribution varies, the wave-length of maxi- 

 mum response will also vary. 



APPLICATION OF SELECTIVE DETECTORS 



We may now ask the general question: What are the value and range of 

 applicability of selective detectors, granting that we have quantitative 

 reproducible methods? They are valuable as control instruments for 

 determining irradiation and dosage. The moment the sensitivity as a 

 function of wave-length of a biological phenomenon becomes known, 

 it is often possible to construct a selective detector whose sensitivity curve 

 matches in wave-length extent and distribution the sensitivity curve of 

 the phenomenon. With such an instrument, w^e may evaluate the 

 effectiveness of a given source in producing such a biological effect and 



