[ 99 ] 



# 



IX. Hue Difference and Flicker Photometer Speed, 

 By Herbert E. Ives *. 



IN the first of the writer's papers on the flicker photo- 

 meter some data were given on the speeds of operation 

 of the instrument when the luminosity of the spectrum was 

 measured against a carbon-lamp comparison standard. 

 These speeds, which are critical speeds for the position of 

 intensity match, show a minimum near '58 jjl. In explanation 

 of this minimum it was remarked f : " 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, dis- 

 appears. 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." 



This explanation appeared adequate to the writer, and 

 partly for this reason, partly because no quantitative theory 

 was then available whose verification depended on fuller 

 data, and partly because no flicker photometer then existed 

 which was entirely free from purely mechanical flicker, or 

 abrupt transitions which might in part behave as such, no 

 further experiments were made on this line. Recently, 

 however, Mr. L. T. Troland % has published somewhat fuller 

 data of the same kind, which he explains in a different 

 manner. According to his view the wave-length-speed 

 curve may be interpreted as the reciprocal of the luminosity 

 ■curve, the minimum in the yellow-green indicating the 

 greatest ivhiteness, which he considers as depending on the 

 same underlying process as luminosity. 



This view is so antagonistic to the present writer's ideas 

 on the meaning of luminosity, and on the mechanism of 

 intermittent vision as developed in recent theoretical 

 papers §, that it appeared desirable to secure some addi- 

 tional experimental data, using the new polarization flicker 

 photometer || . These data, which are given below, appear 

 to substantiate the theory upon which the experiments were 

 based. 



* Communicated by the Author. 



t " Photometry of Lights of Different Colours," Ives, Phil Mag. July 

 1912, p. 167. The italicizing is added in the quotation. 



\ " Apparent Brightness, its Conditions and Properties," Troland, 

 Illuminating Engineering Society Convention, Sept. 1916. 



§ " Theory of the Flicker Photometer," Ives & Kingsbury, Phil. Mag. 

 Nov. 1914, p. 708, and April 1916, p. 290. 



|| u A Polarization Flicker Photometer, and some Data of Theoretical 

 Bearing obtained with it," Ives, Phil. Mag. Apr. 1917, p. 360. 



H2 



