L52 Mr. H. E. Ives on the 



of brightness methods, and by the determination of critical 

 frequency of flicker. But such investigations have been 

 carried out by different observers, using different conditions 

 of illumination and different colour standards, with instru- 

 ments whose dimensions were different in essential particulars. 

 And frequently some of the most important conditions have 

 not been stated. Prominent among these should be mentioned 

 the illumination and the size of the photometric field. 



It is the object of the present studies to obtain data for the 

 solution of this problem. For this purpose comparisons will 

 be made between the various methods. The different factors 

 which influence the relative luminosity of colour will be 

 systematically varied as each method is studied. The same 

 conditions will, however, be maintained throughout, whereby 

 satisfactory comparisons of the different methods will be 

 possible. Attention will be devoted chiefly to the most 

 promising lines of investigation, and ground covered by 

 previous workers will not be touched unless coordination 

 of their work with the present work is found impossible. 

 Problems incidental to the main questions will be investi- 

 gated where necessary, but the ultimate object to be held in 

 view will be the determination of the most satisfactory 

 method of measuring luminosity where differences of colour 

 exist. 



Existing Method* of Jleterocliromatic Photometry, 



The chief methods of light measurement which have been 

 applied to differently coloured lights are : — 



1. The method of equality of brightness. — The two lights 

 illuminate the two parts of a photometric field. The relative 

 intensities of illumination from the two lights are changed 

 until the two differently coloured fields give a sensation 

 of equal brightness. The illuminations are then called 

 equal. 



2. The method of visual acuity. — Two illuminations are 

 equal when the same fineness of detail is just distinguishable 

 by each. 



3. By critical frequency. — Two illuminations are equal 

 when the flicker produced by rapid alternation of one illumi- 

 nation with black disappears at the same speed of alternation 

 as for the other when similarly alternated. 



4. The flicker photometer. — Two illuminations are equal 

 when upon rapidly alternating one with the other no sen- 

 sation of flicker results, the speed of alternation being such 

 that the slightest change of either illumination produces a 

 fiicker. 



