254 proceedings: philosophical society 



indicate that the total X-ray energy emitted, including both that of the 

 characteristics and of the general radiation, is approximately propor- 

 tional to the third power of the potential, or that the efficiency is pro- 

 portional to the square of the potential. The actual value found for 

 the efficiency agrees with that found b.y other observers using the same 

 method. 



A consideration of the results obtained by the ionization and bolom- 

 eter methods indicates that only a fraction of the X-ray energy absorbed 

 by a gas is actually used in the production of ions. The fact that the 

 amount of energy absorbed per ion produced is a variable quantity 

 brings into question the validity of the ionization method of measuring 

 X-ray energy. There is need of further detailed investigation of this 

 particular point. 



Discussion: This paper was discussed by Messrs. Sosman and 

 Webster. 



The 801st meeting was held at the Cosmos Club, February 16, 1918; 

 President BurCxESS in the chair; 41 persons present. 



JNIr. H. E. Merwin presented a paper on Comj)lem.entary colors and 

 the. properties of pigmeyits. The paper was illustrated by samples of 

 pigments. 



Consideration must be given to refractive index, pleochroism, and 

 shape of grain as well as to the more commonly considered properties 

 of pigments. Diffusing power is determined primarily by refractive 

 index and size of grain. The blue of scattered light is a prominent 

 constituent of some grays and purples. The optimum size of grain of 

 colored pigments depends upon the manner in which the color is diffused 

 — whether by the colored grains or by admixed grains or by a subjacent 

 diffusing surface. 



The refractive index of grains of a black pigment should be equal to 

 the refractive index of the vehicle surrounding the grains. A white 

 pigment should differ as much as possible from its vehicle in this respect. 



Mixing white or black with a colored pigment, causes a shifting of hue 

 which depends upon several factors, one of the chief of which concerns 

 the character of the boundary between light that is strongly absorbed 

 and that which is freely transmitted. 



The coloring efficiency of a pigment in mixtures producing tints 

 may be very different from its efficiency in mixtures producing shades. 

 A given amount of colored material will "go farther" as a glazing color 

 than in mixtures. 



Discussion: The paper was discussed by Messrs. Sosman, Bancroft, 

 and Priest. 



Mr. I. G. Priest then presented a paper on A precision metJiod for 

 producing artificial daylight, which was illustrated by lantern slides. 



Light having a spectral distribution of energy closely approximating 



that of daylight (black body at 5000° abs., sun at the earth V surface or 



sun outside the earth's atmosphere), may be produced by passing the 



■ light from an artificial source (acetylene flame, vacuum tungsten lamp, 



or gas-filled tungsten lamp) through two nicol prisms with a crystalline 



