Photometry of Lights of Different Colours. 159 



the conditions of lighting at nearly those of practical photo- 

 metry, and, by making observations for only a limited time 

 each day, to prevent the eye from becoming fatigued to such 

 an extent as to be noticed by difficulty or discomfort in making 

 readings. 



Summarizing the physiological considerations we find that 

 an investigation of heterochromatic photometry must include 

 studies of the effect of varying the size of the photometric 

 field and of varying the illumination. A complete study will 

 include observations for a large range of illuminations and 

 for several sizes of field at each illumination. The state of 

 adaptation of the eye must be as nearly as possible that 

 holding under the conditions of measurement to which results 

 are to be applied. 



Spectral Luminosity Curves as obtained by Flicker 

 and Equality of Brightness Photometers. 



Previous comparisons of the two photometers. — The most 

 extensive comparisons heretofore made between these photo- 

 meters are those of Dow and Stuhr. Comparisons of less 

 complete character have been made by Millar, Wilde, and 

 others. Dow, recognizing the importance of the physio- 

 logical factors above emphasized, has studied the effects of 

 change of illumination and of field-size. For this purpose he 

 used coloured glasses over his light sources. Comparing, for 

 instance, red and green lights, he found by direct comparison 

 that on decreasing the illumination the relative brightness of 

 the red decreased, the decre;ise being very rapid from '2 metre 

 candle down. With the flicker method this decrease was 

 hardly observable, and the change at '2 metre candle was 

 not shown, although the difficulty of flicker measurements is 

 too great at low illuminations to establish this difference with 

 certainty. On decreasing the size of the field, at a fixed 

 illumination, the brightness of the red increased by equality 

 of brightness ; by the flicker method a much smaller increase 

 was found. These experiments were carried out with the 

 same sized field and the same illumination for each method, 

 so that on that score the results are beyond criticism. They 

 indicate that the flicker method is less susceptible to the 

 influence of changed illumination or size of field. 



Stuhr has compared the methods of visual acuity, direct 

 comparison, critical frequency, and flicker, using coloured 

 glasses. He found the methods by critical frequency and by 

 flicker to yield identical results. These differed both from 



