168 Mr. H. E. Ives on the 



(h) Reproducibility of Measurements. 



Early in the course of the investigation a brief test of the 

 reproducibility of measurements was made. It was then 

 found that measurements made from day to day agreed to 

 within the errors of measurement ; that is, the individual 

 flicker points agreed to within one to five per cent, depending 

 on the part of the spectrum, while the equality of brightness 

 points agreed somewhat less well. The chief object of the 

 investigation at that time being to obtain knowledge of the 

 relative positions of the two kinds of luminosity curves as 

 simultaneously obtained, no further study of the question 

 of reproducibility was made until the great body of curves 

 here plotted were obtained. 



At the conclusion of the measurements by the second 

 observer (M.L.) the writer made a number of check measure- 

 ments, which established a fact that had before been indicated 

 by other occasional measurements, and by the measurements 

 of M.L. This is that the equality of brightness curve may 

 change its position with respect to the flicker curve in course 

 of time. It appears that one's idea or criterion of a match 

 in intensity by equality of brightness may change as com- 

 pared with the flicker criterion, assuming the latter to remain 

 fixed. Thus in the set of measurements given in Plate I. 

 (PI. IV.) the equality of brightness method showed, for large 

 fields, the red less bright in proportion to the blue than did 

 the flicker method. Three months later, after an interval 

 of other work, the writer obtained, under the same conditions 

 as before, curves with the red brighter by the equality of 

 brightness method than by the flicker method ; in other 

 words, the equality of brightness curve had shifted from 

 one side to the other of the flicker curve. The first curves 

 of Plate I. and Plate v. (made on different lamps) show this 

 change of relative position just described. 



As to the amount of change experienced by each curve, 

 the data as obtained do not give as complete an answer 

 as is desirable, because some changes in the distribution of 

 energy in the tungsten lamp took place in the interval 

 of time, due to blackening, etc.; and in the last measure- 

 ments an entirely new lamp had to be used. As the curves 

 stand, they show slight changes in the flicker points, but 

 very large in the equality of brightness ones. Taking 

 into account the changes and differences in the lamps, it 

 appears extremely probable that the flicker luminosity curve 

 remains fixed to within the errors of measurement, while the 

 equality of brightness curve remains fixed to within the errors 



