and Flicker Photometer Speed. 



105 



from white, yellow- white (tungsten lamp), red, and blue. 

 The latter colours are selected, near but not on the spectrum, 

 to correspond approximately to the coloured glasses which 

 were used in the experimental work *. These distances 



26. 



10 



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would be proportional to ^-, if equal hue steps were repre- 



. H 

 sented oy equal distances in the sensation triangle. Actually 



the' latter is not the case, and it becomes necessary to plot 



the equal luminosity mixture point on the colour triangle, 



and then determine, as well as existing information will 



permit, the size of the just noticeable hue step at this point. 



* The blue light, obtained with a blue glass and high efficiency in- 

 candescent lamp, lies much nearer the white centre than the point 

 chosen to represent it, but the very wide slit necessarily used in this 

 part of the spectrum, with the consequent impurity, leaves spectrum and 

 blue comparison light probably in proper relative positions. 



