150 Mr. H. E. Ives on the 



3. Requirements of a good method of measurement. 



a. Sensibility. 



b. Summation quality. 



e. Useful value of quantity measured. 



4. Degree to which these requirements are met by existing methods. 



Equality of brightness and flicker methods meet requirements 

 much better than other methods, with apparent advantage 

 in flicker method of greater sensibility. Different results 

 obtained by these two methods indicate importance of a com- 

 parison between them. 



5. Physiological factors to be considered. 



a. "Purkinje" and "yellow spot" effects. Hods and cones. 



b. Colour blindness. 



c. Adaptation and fatigue. 



Spectbal Luminosity Curves as obtained by Flicker and 

 Equality of Brightness Photometers. 



1. Previous Comparisons. Dow, Stuhr, Millar, Wilde. 



Apparent differences in the magnitude of Purkinje and yellow 

 spot ell'ects with the two photometers. 



2. Apparatus and Procedure. 



Arrangements for securing monochromatic spectral colours at 

 various illuminations and with various sizes of the field of 

 view. Details of measurements. 



3. Measurement. 



a. Sensibility. 



Greater sensibility by the flicker method. Flicker speeds. 



b. Reproducibility of measurements. 



Greater reproducibility by the flicker method. 



c. Effect of changing illumination. 



" Purkinje effect " by equality of brightness; reverse effect by 

 flicker. 



d. Effect of changing field size. 



" Yellow spot " effect at low illumination by equality of 

 brightness, reverse of this effect by flicker. 

 c. Relative positions of flicker and equality of brightness curves. 

 Nearest each^other at high illuminations and with small 



fields. 

 A reas of two kinds of luminosity curves different. 

 /. Comparative results obtained by different observers. 



4. Theoretical Considerations. 



Significance of Purkinje and opposite effects. Bearing- on theory 

 of rod and cone action. Insufficiency of present theories. 

 Future work indicated. 



5. Bearing upon Practical Photometry. 



Introduction. 

 A NY light may be defined in terms of hue, saturation, 

 JTJL. and luminosity. Practical photometry is concerned 

 primarily with luminosity. But that quality of light cannot 

 by simple means be separated from the other two. The eye can 

 equate but cannot appraise. Hence the eye is on uncertain 

 ground whenever it attempts to compare the luminosities 



