204 HOWARD E. PULLING 



although as a matter of fact it is well known that it is only valid 

 for a relatively short range of intensities. Such preparations 

 may be "sensitized" 52 for almost any portion of the spectrum, 

 even including the shorter of the infra-red waves, while the 

 11 non-developed " papers cannot be thus sensitized in any way 

 now known. It must not be forgotten that these sensitized 

 preparations also exhibit curves and not bands of sensitivity 

 and some of the curves have more than one maximum. Be- 

 cause such sensitized plates cannot be exposed until a certain 

 amount of darkening results, their use is attended with consider- 

 able difficulties through the uncertainty in the comparison of 

 the developed plate with standard tints. 



Photo-sensitive reactions that occur in solution or in the 

 gaseous condition have been utilized as actinometers but their 

 use is attended by all the uncertainties encountered in the use 

 of silver-chloride-gelatine papers and does not have the off- 

 setting advantage of simplicity, for the solution or gas must be 

 chemically analyzed by more or less convenient means to de- 

 termine the extent to which the reaction has proceeded. A 

 number of these have been proposed but since their curves of 

 wave length sensitivity and behavior under different conditions 

 of radiation intensity are not known, they are at present useless. 



Of the remaining actinometric devices the photo-electric cell 

 may be noted as a valuable instrument when properly con- 

 structed and in competent hands. The selenium cell has been 

 suggested but Pfund 5 ' has found that the position of the maximum 

 sensitivity varies with the intensity of the incident light so that 

 it appears unpromising. 



The alkali photo-electric cells, such as have been developed 

 by Elster and Geitel have presented serious difficulties, which 

 are, however, being overcome as their principles become better 

 understood. Cells with high vacua or containing helium or 



52 See files and current numbers of The British Journal of Photography ; The 

 Photographic Journal; Zeitschrift fur wissenschaftliche Photographie, Photo- 

 physik und Photochemie. 



63 Pfund, A. H., Application of the selenium cell to photometry. Phys. 

 Rev. 34: 370-380. 1912. 



