364 Mr. H. E. Ives on the 



perhaps the influence o£ the surrounding field, enter in the 

 latter case to cause deviations from the simple conditions. 

 At any rate the simple law which it was hoped to find does 

 not hold for the flicker photometer. 



The actual effect of this deviation from the simple law is 

 to cause apparent constancy of the flicker curve at moderately 

 high illuminations, since the changes over a long range of 

 intensities are of nearly the order of the error of reading. 

 It must not be forgotten too that these data are all for a 

 large photometric field. Data for a small field are given in 

 fig. 5 by the dotted lines. Their much greater constancy of 

 value is noticeable. 



A similar test of the method of equality of brightness 

 was made by plotting both the writer's data and Kcenig's 

 in terms of log I against log. slit-width, and some iso- 

 chromatics were tried. Here again long curves were obtained. 

 If any simple law holds with regard to these phenomena it 

 is not this logarithmic relation, which, it may be noted, is of 

 course merely Feehner's law. 



Small Photometric Fields. 



Fig. 6 shows red and blue critical frequency data for a 

 small photometric field (diameter 1°*9). The effect of 

 reducing the field size is to swing the blue line around 

 nearer parallel with the red. which is swung but little. 

 The reversed Purkinje effect will therefore be less, a condition 

 similar to that produced in the flicker photometer by the 

 same change of field size. 



The most interesting point shown in the figure, however, 

 is the behaviour of the bine at very low illuminations. The 

 violent change of direction which occurs with a large field 

 either is absent or is pushed down to a lower illumination 

 than here tried. The fovea behaves toward blue light just 

 as the larger area of the retina behaves toward red. This is 

 in line with the finding of Koenig that the fovea is blue- 

 blind. It is here found to be not strictly blue-blind but 

 blind to the gray sensation which replaces blue at low 

 illuminations for a larger or peripheral field. There is also 

 some indication that the red small field curve tends to turn 

 at low illuminations in somewhat the manner of the blue 

 curve for large fields. The bearing of these points on colour 

 vision is discussed shortly. 



An outstanding difference between Haycraft's work and 

 the present research may be noted here. While his results 

 are qualitatively explained by the data of fig. 4, these data 

 would not give his curves. Blue light attains a fixed critical 



