298 Messrs. H. E. Ives and E. F. Kingsbury on the 



be desired. The best correspondence would call for a value 

 of 8 of about '001, but even with this small value the slope 

 of the theoretical line is too great. We would again point 

 out that while the theoretical work is done for the case of 



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Phenomena of superposed steady and flickering illuminations. Dashed 

 lines calculated from theory. Full lines, experimental values, showing- 

 varying- inclinations as suggested by theory, but actual slope and magni- 

 tudes not as calculated from simplest assumptions. 



gradual transitions, the experimental is done with abrupt 

 transitions. In addition, our assumption that the diffusivity 

 varies with the logarithm of the mean illumination may be 

 wrong. It is, for instance, possible that the -diffusivity is 

 a function of an illumination intermediate between the 

 maximum and the mean. Mechanical defects of the photo- 

 metric field may as well be active as disturbing causes. 



These points are, however, secondary to the fact that the 

 theory accounts for and, in fact, has prophesied the pre- 

 dominating characteristics of the cases considered. In both 

 it is evident that information of some value would be 

 furnished by performing the experimental work with appa- 

 ratus giving the exact kind of transition which is treated 

 by the theory. The experimental difficulties of obtaining 

 a sine-curve transition from one illumination to another are, 

 however, rather formidable, and we have not felt the slight 

 additional support which might be furnished to the theory 

 to warrant undertaking the great additional labour. 



