650 Mr. J. S. Dow on a Form of 



appears to oscillate with it. A band o£ flicker is thus pro- 

 duced, and the field of view assumes the appearance shown 

 in fig. 5. 



Fig. 5. 



(a) (b) 



The observer may then judge the position of photometric 

 balance by observing the cessation of flicker in the band. Or 

 he may drive the motor so fast that all flicker disappears and 

 the intermediate band merely assumes a colour intermediate 

 between that present in the two adjacent portions of the field 

 of view. He may then use this intermediate colour to assist 

 him in his decision as to the exact point when all three 

 sections of the field of view appear equally bright. 



The question may now be raised whether, when using this 

 instrument, the results of using the Equality of Brightness 

 and Flicker methods is the same. 



The author has found that for widely different colours and 

 for certain portions of the retina this is not the case *. Under 

 the conditions present in this instrument, however, the agree- 

 ment was very close unless the colour difference was very 

 marked indeed. This is illustrated by the results shown 

 in the following : — 



Nature of Lights 

 compared. 



Eesults by Flicker 

 method. 



Result by Equality of 

 Brightness method. 



White to White 



1-031 

 0-911 

 1-21 

 2-23 



1-034 

 0-900 

 1-08 

 209 



White to Apple-Green . 



(impure) 

 White to deep Green ... 



White to deep Ked 



Proc. Phys. Soc. vol. xx. 



