166 Mr. H. E. Ives on tl 



ie 



(d) Effect of changing field-size on each method, for 



various illuminations. 



(e) The relative positions of the luminosity curves by 



the two methods at various illuminations and field- 

 sizes. 

 (/) Comparison of luminosity curves obtained hj dif- 

 ferent observers. 



[a) Sensibility. 



A measure of the relative sensibility of the two methods is 

 obtained by taking the mean error of setting. In fig. 2 

 (PI. III.) are plotted the mean errors for five observers at 

 250 illumination units for various points in the spectrum, 

 from the deep red at '66 /n to the green at *51//,. The mean 

 flicker errors are given by the full line, the mean equality 

 of brightness errors by the dashed line. In fig. 3 are 

 given the same quantities as obtained from observations at 

 10 illumination units. Several facts are here brought out 

 clearly. First, the flicker method is for all parts of the 

 spectrum several times as sensitive as the equality of bright- 

 ness method at the higher illumination, the relative sensibility 

 differing for different observers, but always favouring the 

 flicker method. Second, the difference in sensibility between 

 the methods is greater at high illuminations than at low. At 

 the lower illumination the equality of brightness sensibility 

 becomes greater, the flicker sensibility less. Third, the sensi- 

 bility by the flicker method is less toward the ends of the 

 spectrum, where the difference in hue between the spectrum 

 colour and the standard lamp colour is greatest. 



It is to be noted that while all five observers made readings 

 by the flicker method which compared very closely to the 

 accuracy of those made by the two most experienced obser- 

 vers (H.E.I, and M.L.) two of the others (by whom settings 

 were made for the first time) averaged five or ten times the 

 error by the equality of brightness method as by the other. 

 This illustrates the great superiority of the flicker method as 

 a method of measurement with observers not used to making 

 matches between lights of widely different colour. 



In connexion with the question of sensibility may be given 

 the data on the speeds used with the flicker photometer. As 

 is well known, sensibility varies with the speed, being less 

 at high speeds. In fact, with high enough speed all flicker 

 vanishes, no matter what the difference in illumination from 

 the two sources under comparison. In these experiments the 

 speed was always adjusted to the lowest value at which 

 flicker could be made to disappear. This was then read by 



