SUNLIGHT AND ITS MKASTHiKMKNT 205 



argon are much to be preferred to those containing reactive 

 gases because the ionization in the latter type upon exposure 

 to intense radiation produces marked irregularities of behavior. 54 

 Like all instruments in the actinometric class they exhibit a 

 curve of sensitivity and this curve must be known or the results 

 obtained from their use in different portions of the spectrum 

 are meaningless. Moreover, it is at present impossible to con- 

 sciously prepare two cells even approximately alike in short 

 or long wave limit of sensitiveness, or in spectral position or 

 degree of maximum sensitiveness, so that the curve of sensitivity 

 must be known for each individual cell. 55 To further com- 

 plicate the problems connected with their use it has been found 

 that their curves change with age of the cell so that the curve 

 of sensitivity must be investigated at sufficiently short in- 

 tervals. 56 The rate of change of these curves depends to some 

 extent upon the degree of evacuation of the cells, full gas cells 

 changing more rapidly than those with high vacua. Because 

 the photo-electric effect occurs at the surface of the metal, 

 advances in methods for preparing these surfaces should be 

 attended by greater uniformity of cells; at present surfaces 

 very carefully prepared at the same time and from the same 

 materials yet show decided differences in behavior, although 

 with proper standardization this objection is not serious. If 

 the cells are properly constructed, the photo-electric current 

 appears to be directly proportional to the intensity of radiation, 57 



54 Elster, J., and H. Geitel, Uber erne notwendige Vorsichtsmassregel bei 

 der Photometrie sehr grosser Lichtstarken vermittels hochempfindlicher Al- 

 kalimetallzellen. Physik. Zeit. 15: 610-611. 1914. 



35 Ives, H. E., Wave length sensibility curves of potassium photo-electric 

 cells. Astrophys. J. 40: 182-186. 1914. 



56 Compton, K. T., and O. W. Richardson, The photoelectric effect II. 

 Phil. Mag. (6) 26: 549-567. 1913. 



57 Kunz. J., The law of photoelectric photometry. Astrophys. J. 45: 69- 

 88. 1917. 



Coblentz, W. \\. , Instruments and methods of radiometry III. The photo- 

 electric cell and other selective radiometers. U. S. Bur. Standards Sci. paper 

 319. 1919. 



Ives, H. E., The variation with time of the characteristics of a potassium 

 photo-electric cell as to sensibility according to wave length. Astrophys. J. 

 46: 241-248. 1917. 



