200 HOWARD E. PULLING 



case of preparations that are " developed" before the degree of 

 darkening is judged. In this latter case (as with "plates") 

 the relation becomes exponential and is termed Schwarzschild's 

 rule. 51 It must be remembered that these expressions are 

 only approximations to an unknown law and do not hold for 

 very high or very low intensities. 



Since silver-chloride-gelatine reactions, in common with other 

 photochemical reactions, vary in their rate when exposed to 

 radiation of different wave lengths it is obvious that unless the 

 energy curve of the incident radiation is of the same form in 

 all cases for which it is desired to measure the intensity, the 

 rate of reaction alone will tell nothing about the intensity of 

 the entire beam of light. Line A in figure 5 represents the curve 

 of sensitivity of a silver-chloride-gelatine preparation (of course 

 it is not the curve for any such preparation) to radiation at 

 different wave lengths. The ordinates accordingly represent 

 degrees of darkening in equal times when the intensity of radiation 

 in any wave length is equal to that at any other. 



If we imagine a beam of radiation to be incident upon this 

 silver-chloride-gelatine actinometer and to be of equal intensity 

 in all wave lengths between about 0.29 n and about 0.53 u (the 

 approximate limits of sensitivity of the instrument) it will be 

 at once apparent that the relative effects of the various kinds 

 of waves in darkening the paper per unit of time will be pro- 

 portional to the heights of the ordinates of curve A. Accordingly 

 the total darkening of the paper in unit time by such a beam 

 will be proportional to the area enclosed between the curve A 

 and the base line. The darkening by such a beam at any ef- 

 fective portion of the spectrum smaller than the entire effective 

 portion will be proportional to the area enclosed between the 

 base line and the curve A between the ordinates erected at the 

 wave lengths bounding the portion of the spectrum considered, 



51 Schwarzschild, K., Leber abweichungen von Reciprocitatsgesetz fur 

 Bromsilbergelatine. Photograph. Correspondenz 36: 109-112. 1899. 



Renwick, F. F., Some deductions from Schwarzschild's rule. Phot. J. 56: 

 11-13. 1916. 



Draper, J. \Y '., On some analogies between the phenomena of the chemical 

 rays and those of radiant heat. Phil. Mag. (3) 19: 195-210. 1841. 



