Photometry of Lights of Different Colours. 173 



fixed character of the flicker curves, gave the equality of 

 brightness curves as substantially the same for all field-sizes. 

 Therft is, therefore, good ground for believing that at the 

 highest illumination here used the effects of changing field- 

 size are small or absent by both methods, although the in- 

 herent uncertainty of the equality of brightness method may 

 indicate changes even at high illuminations. At lower illu- 

 minations the changes are of an entirely different order of 

 magnitude from the uncertainties of judgment, and so can 

 be described positively. It appears then that the Purkinje 

 and yellow spot effects are two phases of the same low 

 illumination phenomenon. In brief, at low illuminations for 

 large fields there occurs the Purkinje effect by the equality 

 of brightness method ; the reversal effect by the flicker. At 

 these illuminations a decrease in the size of the field decreases 

 the effects. 



(e) Relative positions of flicker and equality of 

 brightness curves. 



Plates in. and in. a (PI. Y.) give the relative positions of the 

 two kinds of curves for three field-sizes and four illuminations. 

 The most striking phenomenon is the large difference in the 

 curves at low illuminations. At high illuminations the curves 

 approach each other. In the case of the first observer, 

 practical coincidence of the two curves is obtained for the 

 middle-sized field at the highest illumination. In the case 

 of the second observer (Plate in. a) this coincidence does 

 not occur, the curves merely being much nearer together 

 and (as test measurements at higher illuminations showed) 

 they have reached a " steady state. " With regard to the 

 different relative behaviour of the two curves with the two 

 observers, the remarks made on "reproducibility" and the 

 comparative results of five observers given below, are apropos. 

 With both the observers whose curves have so far been shown, 

 there has occurred during the progress of the work a change 

 of criterion with the equality of brightness measurements, 

 and in the case of the five observers some show the equality 

 of brightness maximum to one side, some to the other of the 

 flicker. The only conclusions that can be drawn are that 

 the two curves change from wide disagreement at low illumi- 

 nations to a much closer agreement (in position of maximum) 

 at high illuminations — the closeness of the agreement being- 

 difficult to determine because of the uncertainty in the real 

 position of the equality of brightness curves. One point of 

 significance is that although with both observers the two 



