852 Mr. H. E. Ives on the 



The results were as follows : — 



By parts. Total. 



H.E.I. I 691 695 



H.E.I. II 742 743 



P.W.C. 1 773 785 



(Slight changes in the position of the variable screen and 

 comparison lamp cause the different magnitude of the results 

 in the arbitrary units given.) The greatest difference is in 

 the case of the less practised observer and amounts to 

 1^ per cent. This lies within the range of his errors of 

 setting. 



The conclusion is drawn that under the conditions of high 

 illumination and small field size specified, the arithmetical 

 sum of the parts as measured agrees with the measured value 

 of the whole, by the flicker method, for observers of normal 

 colour vision. 



With the proof of this property of the flicker method of 

 photometry is concluded the set of tests which were planned 

 at the beginning of the investigation. A general summary 

 of the characteristics of the flicker method follows : — 



1. It surpasses all other photometric methods in sensibility 

 and reproducibility in the presence of colour difference. 



2. It agrees at high illuminations with the equality of 

 brightness method, when the latter is freed from the psycho- 

 logical uncertainties inherent in its use. 



3. It measures at high illuminations what may fairly be 

 termed the true brightness. 



4. Brightnesses measuring equal to the same measure 

 equal to each other and the sum of the measurements of the 

 parts is equal to the measurement of the whole. 



In the opinion of the writer the flicker photometer should 

 be adopted as the standard instrument for making hetero- 

 chromatic comparisons in the standardizing laboratory. 

 Before, however, this can be done, the standard conditions 

 of illumination and field-size should be agreed upon, and, 

 most important of all, the normal luminosity curve of the 

 eye should be determined from observations on a large 

 number of individuals. This done, not only will it be possible 

 to determine which individuals should be chosen to make 

 such comparisons, but also, in virtue of the various qualities 

 of the flicker photometer, it will be possible to correct to 

 normal the results of any observer whose luminosity curve 

 is known. The next paper of this series will be devoted to 

 a discussion of standard conditions for heterochromatic 



