Photometry of Lights of Different Colours. 171 



toward the red, or exactly opposite. Also to be noted is 

 that with the smallest field these shifts are less marked. 



Examining the curves more in detail, several interesting 

 points appear. Taking first Plate I. (H.E.I.) and examining 

 the quality of brightness (lower) curves, it appears that the 

 effect of decreasing illumination is two-fold : first occurs a 

 broadening of the curves (increase of area), then an increase 

 on the blue side, which results in a shift of the whole curve 

 toward the blue. This may be interpreted as an increase on 

 the red side, followed by an increase on the blue side, the 

 latter becoming more marked at lower illuminations. With 

 decrease in the size of the aperture these changes are much 

 less, which is in accordance with the oft encountered 

 statement that the Purkinje effect is absent for very small 

 fields. 



The flicker curves exhibit a plain shift toward the red, 

 with not much difference for the different field-sizes until 

 the lowest illumination is reached. At this lowest illumina- 

 tion there appears a drop on the red side, most marked for 

 the largest field-size. In consequence of this the area of 

 the curve is less and its maximum shifts again toward the 

 blue. 



Consulting Plate i.a we find the same general phenomena 

 exhibited, a shift toward the blue for equality of brightness, 

 toward the red for flicker, with decrease in illumination. 

 Some differences between the results of the two observers 

 are to be noted. Chief among these is that the correspond- 

 ing changes in the curves take place at different illumina- 

 tions and field-sizes from those of the first observer. The 

 lowest measurement, for instance, was made at an illumi- 

 nation where the writer could secure no reliable flicker 

 measurements. 



As to the equality of brightness measurements, it is to be 

 noted that the increase of luminosity on the red side of the 

 curve is absent with the large aperture, slight with the middle 

 aperture, and pronounced with the smallest aperture. The 

 smaller increase of the blue sensitiveness or Purkinje effect 

 with the small field is not only present, but because of the 

 enhanced red sensitiveness the maximum of sensibility for 

 low illuminations actually shifts toward the red, somewhat 

 as the flicker curve for the same field-size and illumination. 

 As a consequence of these facts the curves of this plate do 

 not show a broadening of the curves, preceding the shift 

 toward blue. The small aperture lowest-illumination curve 

 seems to indicate a drop in red sensitiveness, or a reversal 

 similar to the one occurring with the large field flicker curves 



