848 Mr, H. E, Ives on the 



easily happen— in the case o£ a purple composed of reel and 

 blue light — that increase o£ illumination reduces the detail- 

 revealing power o£ the eye, although for the component 

 parts of the acting light the normal relation between visual 

 acuity and brightness holds. The visual acuity curve of 

 fig. 1 is therefore analysable as a -combination of the acuity- 

 brightness relation and what may be termed a quality factor. 

 It is a perfect example of a method of measurement in which 

 axiom 1 is valid, but not axiom 2. Needless to say, the 

 existence of such a quality factor effectually disposes of 

 the method of visual acuity as a method of measuring 

 brightness. 



The differences between the remaining three methods are 

 probably due to more obscure psychological causes. In the 

 case of critical frequency the ease with which area differences 

 may be produced has been pointed out. The manner in 

 which the equality of brightness curves are disturbed by 

 varying the comparison standard is sufficient reason for 

 regarding without surprise the differences which have been 

 found between these areas and those by the flicker method. 



The crucial test of axiom 2 appears to be, in the light of 

 all previous work, reduced to its test by the flicker method. 

 Such tests have been made by Whitman *, and later by 

 Tufts |. Since the latter's was more complete, the experi- 

 mental test which is described below was modelled upon his, 

 with, however, certain very essential modifications. A brief 

 account of his experiment is necessary. 



Two identical incandescent lamps were chosen, one so 

 placed as to illuminate a white surface before the slit of a 

 spectrometer, the other so placed as to illuminate a rotating 

 sector in front of the telescope lens. The latter lamp was 

 placed on a photometer track so as to be movable. The 

 slit-width was held constant and the brightness of the 

 spectrum measured at several points by the flicker method. 

 At the end the prism was removed, and the brightness of the 

 total light through the spectrometer was measured against 

 the movable lamp. This was found to be within a few per 

 cent, the same as that indicated by the area of the luminosity 

 curve. 



Further experiments showed the area of the luminosity 

 curve to be unaltered by observing through a coloured glass, 

 as it should not be if axiom 2 holds. When, however, a 

 deep cobalt blue glass was used a large discrepancy was 

 found. Certain other observers, who were less red-sensitive 



* Whitman, Pliys. Eev. iii. p. 241 (1896). 

 t Tufts, Pliys. Rev. xxv. p. 433 (1907). 



