Photometry of Lights of Different Colours. 165 



(2) Effect of changing size of field, at each of ten illuminations. 



(a) On the direct comparison curves. 



(b) On the flicker curve. 



(c) The relative position of the two curves due to the 



change. 



Among the curves are some showing bad points and some 

 which, as the text indicates, do not exactly agree with those 

 taken at other times. These imperfect curves (practically all 

 " equality of brightness " curves) are used because even with 

 their defects they show a proper relation to the preceding and 

 following ones, while curves made at a later date purposely 

 to replace them would correspond to a changed criterion. 

 Attention must be called to the fact that in the measurements 

 involving physiological variables, it is impossible to exactly 

 reproduce conditions holding at one time ; hence data must 

 occasionally be used for which, were it possible, one would 

 by preference substitute other determinations. The curves 

 selected have been most carefully chosen so as to represent 

 most clearly the various phenomena brought out by the 

 measurements. 



Every effect to which attention is called and every con- 

 clusion is supported not only by the curves shown in this 

 article, but also by others made at intermediate illuminations. 

 Further, to avoid any possibility of error due to change in 

 the light source between measurements, check settings at one 

 time have been made to verify (at least qualitatively) those 

 phenomena which are indicated by the differences between 

 curves made on various days. 



The data here presented consist principally of three sets : — 



1st. Measurements made by the writer, chiefly during a 

 period of about eight weeks in May, June, and 

 July, 1910. 



2nd. Measurements made by Mr. Luckiesh during July 

 and August. 



3rd. Measurements made at selected points for one field- 

 size by five observers of normal colour vision during 

 two weeks in September. 



These will now be considered from various points of view. 

 In turn will be taken up : 



(a) Relative sensibility of the flicker and equality of 



brightness methods. 



(b) Reproducibility of measurements. 



(c) Effect of changing illumination on each method for 



each field-size. 



