172 Mr. H. E. Ives on the 



with the writer. These secondary effects occur at the lowest 

 illumination, where measurements are difficult, and thus 

 cannot be established so well as the high illumination 

 phenomena. 



The flicker curves show similarly different effects due to 

 changed illumination at different field-sizes, for while all 

 show an increase of red sensitiveness for low illumination, 

 the effect is most marked with the large field. There is no 

 apparent reversal of the direction of the shift, although at 

 the lowest illumination there is an increase of blue sensitive- 

 ness accompanying the increase of red, with the largest 

 aperture. This is on a parallel with the drop in red sensi- 

 tiveness at low illumination for large field in the writer's 

 case. 



One point is not well shown on these curves (which were 

 selected to illustrate extreme cases), viz., that the shifts 

 plotted occur chiefly at low illuminations. The high illumi- 

 nation curve represents very closely the position of the curves 

 for a large range of illumination. Comparative fixity is 

 found, in fact, from about 70 I.U. up. 



The highest illumination (270 units) was the highest it 

 was practicable to work at with the tungsten lamp. Some 

 measurements made at about twice this illumination by 

 viewing an acetylene flame directly showed that no appre- 

 ciable change in the curves took place beyond this point. 



(d) Effect of changing field-size. 



Plates II. and II. a (PI. IV.) present the luminosity curves at 

 each illumination, as obtained with each size of field. The chief 

 facts are, first, that at the high illuminations change of field- 

 size has practically no effect on the flicker curves. Second, 

 decreasing the size of field increases the red sensitiveness of 

 the equality of brightness method (at practically all illumi- 

 nations) but decreases the red sensitiveness by the flicker 

 method. In other words, just as there is a reversed Purkinje 

 effect, with the flicker method, so there is a reversed " yellow 

 spot effect." In the case of this effect, as with the changes 

 due to changed illumination, it was necessary for the second 

 observer (Plate n.a) in order to experience these changes, 

 to go to lower illuminations than did the first observer. 



It is questionable whether the changes plotted in the high 

 illumination curves for the equality of brightness methods 

 are true " yellow spot effect " changes, or are due to the 

 uncertainty of the method. Some check curves made two 

 months after the first of Plate II., while exhibiting the same 



