Photometry of Lights of Different Colours. 167 



the tachometer and reduced to cycles per second. In fig. 4 

 (PL III.) are given the speeds as used by the different ob- 

 servers from end to end of the spectrum and for the two 

 illuminations. The data show that much lower speed is 

 necessary for low illumination, and that the ends of the 

 spectrum require higher speed than the middle. These facts 

 readily fit in with our knowledge derived from other sources 

 and with such theory as we have of the action of the flicker 

 photometer. The eye is more sensitive to flicker at high 

 illumination than at low, hence the greater sensitiveness of 

 the flicker method and the higher speed necessary at high 

 illuminations. In order to compare lights of different colours 

 it is necessary to attain such a speed that the colour flicker, 

 due to difference in hue, disappears. It is therefore to be 

 expected that at the ends of the spectrum where the hue is 

 most different from the comparison lamp, a higher speed is 

 necessary, and with this higher speed goes decreased 

 sensibility. Whether the change in sensibility is exactly 

 what the change in speed would occasion, or whether the 

 change in speed is conditioned by hue difference alone, are 

 points for future study. 



In fig. 5 (PI. III.) are plotted some preliminary data upon 

 flicker speeds and sensibility under varied conditions of field- 

 size and direction of vision. The two upper curves show the 

 speeds (at 270 illumination units) for the largest and smallest 

 fields used. The large field requires the higher speeds ; in 

 other words, it is more sensitive to flicker. The lowest curve 

 gives the speeds used for observation by peripheral or averted 

 vision. To obtain these the smallest aperture was displaced 

 to one end of the total available opening, and attention was 

 fixed upon a small pin-hole (in which no flicker was visible) 

 at the other end, about 8° distance, flicker being observed 

 at the small aperture. The most striking feature is the 

 much lower speed, corresponding to that required for lower 

 illumination. Above are plotted the mean errors for 

 observation by peripheral vision. These are much larger 

 than by central vision. Later, the theoretical bearing of 

 these points will be discussed. It is hoped ultimately to 

 follow them up with more extended investigations of the 

 retinal field, not only for speed and sensibility, but also for 

 the character of the luminosity curves. These have, however, 

 been deferred until some of the points of more direct interest 

 to photometry are studied, although the complete explanation 

 of the phenomena encountered will probably be brought out 

 by study of extreme conditions, snch as those holding in 

 peripheral vision. 



