PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



SOCIETIES 



THE PHILOSOPHICAL SOCIETY OF WASHINGTON 



The 777th meeting was held on October 28, 1916, 'at the Cosmos 

 Club. President Briggs in the chair; 50 persons present. The min- 

 utes of the 776th meeting were read in abstract and approved. The 

 Chair informed the meeting of the death on the morning of October 

 28, 1916, of Mr. Cleveland Abbe, a charter member of the Society. 



Mr. A. H. Taylor presented an illustrated paper embodying the 

 results of an investigation in collaboration with Messrs. E. C. Crit- 

 tenden and F. K. Richtmyer on A normal eye for the photometry of 

 lights of different color. Individuals in general differ in their judgment 

 of the relative brightness of lights which differ in color. Consequently, 

 in order to assign definite values to such lights it is necessary to es- 

 tablish some common basis of comparison and to provide means for 

 reducing to this common basis the results obtained by different- ob- 

 servers. For measurements of incandescent lamps it has been pro- 

 posed to test observers by having them determine the relative trans- 

 missions of a reddish-yellow (potassium bichromate) solution and a 

 blue-green (copper sulphate) solution. This paper is a report of a 

 trial of this method. For large color differences a flicker photometer 

 must be used, and the size and brightness of the photometric field 

 must be specified, to make the results definite. The conclusion 

 reached was that the method proposed does give a practical means of 

 establishing a normal eye and of reducing to the normal value the 

 results obtained by any group of observers. 



The paper was discussed by Mr. Middlekauff, who referred to 

 the intercomparisons at various laboratories of a series of glasses for 

 the purpose of establishing a basis for the calibration of other color 

 screens. The use of the flicker photometer necessitates no change in the 

 photometric standards already adopted by the Bureau of Standards. 



Mr. W. W. Coblentz then spoke on The relative sensibility of the 

 average eye to lights of different colors, giving the results of an investi- 

 gation by himself and Mr. W. B. Emerson. The paper was illustrated 

 by lantern slides. In the present investigation the methods were 

 practically the same as those used by previous experimenters. In the 

 visual measurements, the spectral light was compared with a stand- 

 ard white light by means of a flicker photometer, also by an equality- 

 of-brightness photometer. The source of white light was a standard- 



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