748 Mr. H. E. Ives on the 



in the latter a bright surrounding field necessitated a higher 

 speed for the rotating disk. Although iio certain increase in 

 sensibility appeared, considerably greater comfort was felt 

 iu working with the larger area of retina active, which would 

 recommend that condition for accurate work if a small field 

 is used* 



At low illuminations with a large field where the Purkinje 

 effect appears in the equality of brightness method, and the 

 opposite effect by the flicker method, an interesting question 

 is whether these effects are to be connected with the exci- 

 tation of a large retinal area or only with the area which is 

 concerned with the actual comparisons of brightness, or 

 detection of flicker. In short, at low illuminations, will 

 surrounding a small photometric field with a large bright 

 area be equivalent to Using a large photometric field ? The 

 answer is in the negative, for while bright surroundings 

 shift the equality of brightness curves, the shifts are neither 

 in direction or amount consistently such as would be called 

 Purkinje shifts, and with the flicker method the absence of 

 any shift disposes of the question at once. 



It is evident from these experiments that no reliance may 

 be placed upon an equality of brightness luminosity curve 

 obtained Under such conditions that the psychological 

 element of the judgment of equality of essential inequalities 

 is necessary s The fact that the flicker photometer is free 

 from such uncertainty is a strong argument in its favour. 

 There still remains unsettled, however, the question whether 

 the flicker photometer gives something that may With pro- 

 priety be called the true brightness. In other Words,, does 

 it give what would be obtained by the equality of brightness 

 method with the psychological variables eliminated ? 



This query naturally leads to the consideration of a method 

 of heterochromatic photometry which has been advocated 

 and used in certain cases^ namely that by steps of small hue 

 difference^ or in cascade, as it is sometimes called 4 The 

 principle is that if the hue difference is made small enough 

 it will not disturb the brightness judgment. In establishing 

 incandescent lamp photometric standards of different colour^ 

 this method is particularly feasible, since all intermediate 

 colours are obtainable by voltage variation. 



In order to obtain spectral luminosity curves by this 

 method, two additions Were made to the apparatus as 

 previously described. First a second spectrometer similar 

 to the first, but with its telescope removed, Was added. 

 A first surface platinum mirror, obtained by cathode depo- 

 sition on a piece of glass, which was afterward cut in two 





