Photometry of Lights of Different Colours. 355 



the middle. He plots critical frequencies for constant slit- 

 width, instead of slit-widths or reciprocal slit-widths for 



constant frequency as is here done. It is in this difference 









































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Critical Frequencies of the spectral colours at high, medium, and low 

 intensities, showing relatively greater change for red (K). After 

 Hay or aft. 



of procedure that we find the explanation of the difference 

 between Haycraft's results and those of the present writer. 

 An examination of Haycraft's curves indicates that in all 

 cases the end portions of the spectrum are at illuminations 

 calling for lower critical speeds than the low curve obtained 

 by the writer. Curve C, fig. 1, corresponds to 18 cycles 

 per second, while Haycraft's speeds vary from 19 to about 

 4 cycles per second. After noting this fact an additional 

 critical frequency curve (Curve D) was determined for an 

 illumination less than a tenth that of Curve 0. This shows, 

 with respect^ to the one at 5 I. XL, a pronounced Purkinje 

 shifty in qualitative agreement with Hay era ft. 



Apparently, therefore, the changes which accompany 

 decreasing illumination are first in one direction and then 

 in the other. In searching for some other phenomena of 

 critical frequency with which this might be coordinated, it 

 occurred to the writer that the regions of the Purkinje and 

 the reversed Purkinje effect might be connected with the 



