L60 Mr. H. E. Ives on the 



the results of visual acuity and equality o£ brightness mea- 

 surements, lying between these latter. Different field-sizes 

 — unspecified — were used for visual acuity and other mea- 

 surements, and in no case was attention paid to the absolute 

 illumination. In other words, the physiological factors were 

 not considered. 



P. S. Millar describes measurements of a mercury arc 

 against incandescent lamps in which the illumination varied 

 over a wide range. With equality of brightness photometers 

 the mercury arc measured relatively much brighter at low 

 illuminations : with flicker photometers this effect was absent. 



Wilde describes measurements of a tungsten lamp against 

 a carbon, made with a flicker photometer and with an 

 equality of brightness photometer. The photometers showed 

 a disagreement of several per cent. 



Of these comparisons that of Dow is by far the most 

 complete and scientific, establishing as it does that the two 

 methods give different results for the same size field when the 

 illumination is changed, and that at a fixed illumination they 

 respond differently to changes of field-size. The two points 

 in which Dovv's work is incomplete hold also for the work of 

 the other investigators quoted, while their work is less 

 complete in other ways. First, it is to be noted that the 

 experiments refer to no definite scale of colour, such, for 

 instance, as the spectrum. It is therefore impossible to 

 calculate from his results the magnitude of the effects of 

 varying conditions for any given energy distribution. 

 Second, while the effect of varying either illumination or 

 field-size was investigated, the investigation was not con- 

 tinued to the point of varying both together. There is, for 

 instance, a large range of illuminations suitable for the 

 carrying out of measurements. At each of these illuminations 

 anyone of several sizes of field maybe used. Our knowledge 

 of the relative behaviour of the two methods is incomplete 

 until we know, on some definite colour scale, how these two 

 variables — illumination and field-size — affect the criteria fur- 

 nished by the methods, for all practicable combinations of the 

 variables. The importance of this knowledge may be indicated 

 by considering the character of the differences between the 

 methods as indicated by Dow\s work : — 



The Purkinje effect is less with the flicker method. We 

 also know from the work of Kcenig and Brodhun that the 

 Purkinje phenomenon practically disappears for high illu- 

 minations. The effect of decreasing field-size is (according 

 to Dow) less for the flicker method, but we also know that 

 the Purkinje effect by direct comparison has been found 



