178 Mr. H. E. Ives on tie 



6. The curves are most different in position at low illumi- 



nations with large fields ; nearest together at high 

 illuminations and with small fields. They may under 

 certain conditions coincide, and the mean curves of 

 several observers show close agreement in position 

 of maxima and shape of the two curves at high 

 illuminations, although the areas are not the same. 



7. The curves obtained by different observers show different 



positions for each curve, and different relative positions 

 of the two for high illuminations. At low illumina- 

 tions all observers agree in showing the Purkinje and 

 reversed Purkinje effects above described. 



Theoretical Considerations. 



No satisfactory theory of the action of the flicker photo- 

 meter can be said to exist. What does it actually measure ? 

 We may assume the existence of a " luminosity sense " dis- 

 tinct from the colour sense. We may then form a geome- 

 trical picture of the alternations of luminosity as occurring 

 in one plane, those of hue in another. In the second plane 

 the sensations fuse more quickly, so that the sensation of 

 flicker due to difference of luminosity persists longer than 

 that due to difference in hue. Certain experiments by Tufts 

 on the fatigued eye give some warrant for the assumption of 

 this luminosity sense. The occurrence of colourless after 

 images from coloured stimuli may also support this concep- 

 tion. The luminosity sense may be identified with the black- 

 white element of Hering's theory. But at present it is 

 questionable whether this luminosity sense can be considered 

 as much more than a name used to assist in picturing the 

 action of the flicker photometer. If it should be established 

 that there does exist a separate "luminosity" element, which 

 could be separated from hue by some photometric method, 

 it would still remain to be established that this corresponded 

 to our ordinary idea of brightness. If, for instance, there 

 exists a physiological process called into action both by 

 coloured and uncoloured light, a measure of this would be a 

 measure of a common property. This might be satisfactorily 

 termed " brightness" by a physiologist, but might not satisfy 

 the photometrist who is interested not so much in the ultimate 

 analysis as in things as they appear. 



Dow has given reasons for believing the action of the 

 flicker photometer to be chiefly ascribable to the retinal 

 cones. The rods are known to be more sluggish, to possess 

 greater inertia, so that they are less sensitive to flicker. The 

 fact that the flicker photometer cannot be used at low illumi- 

 nations, and that, as Dow found, the yellow spot and Purkinje 



