Photometry of Lights of Different Cotours. 169 



of measurement only so long as one's memory retains the idea 

 first formed as to the brightness of coloured fields. In course of 

 time the latter curve may experience changes much greater 

 than the change given by the errors of measurement. 



A satisfactory test of reproducibility can only be made 

 with lamps and apparatus standing absolutely unchanged 

 between measurements — a condition which cannot be abso- 

 lutely maintained. Nevertheless, a test approaching this in 

 character has been carried out since the conclusion of the 

 comparative measurements by different observers. Two 

 seasoned " 4- watt " carbon lamps have been used as sources, 

 the apparatus has been clamped rigidly in position, and at 

 intervals of about a week, for a period of two months, flicker 

 and equality of brightness curves have been made by two 

 observers. The middle-sized field was used, at the highest 

 illumination (270 units). 



In fig. 6 (PI. III.) are plotted the mean deviations from the 

 mean, or the mean error of setting, where the mean of all is 

 taken as correct. These show less satisfactory reproducibility 

 by the equality of brightness method than by the flicker method. 

 The difference is not everywhere, however, as great as might 

 be expected from the relative sensibility data and the con- 

 siderations above mentioned. It must, however, be borne in 

 mind that these results are obtained by two very practised 

 observers, whose criterion of equality of brightness had 

 become well fixed long before those measurements were 

 made. With new observers or with observers who rarely 

 made comparisons involving such large colour differences, 

 the equality of brightness method would not have shown up 

 so well. The indications are that with practice one's criterion 

 becomes apparently fixed so that equality of brightness com- 

 parisons can be made with a high degree of reproducibility, 

 though not with as high a degree as the flicker method. 

 Whether this criterion is correct or not is another matter. 



A point which deserves mention is the question of repro- 

 ducibility at low illuminations. There it has been found by 

 experience that reproducibility is much less satisfactory by 

 both methods, and for a pretty clear reason, viz. one's illumi- 

 nation scale at low illumination is not a fixed but a shifting- 

 thing, due probably to the different physiological condition 

 of the observer at different times. A set of curves for three 

 field-sizes, for instance, will be obtained on three successive 

 days and will show a certain relation at a given low illumi- 

 nation. After an interval of several days or any interruption 

 of habits, an attempt to check these may give three curves 

 consistent with themselves but corresponding either to a 



