proceedings: philosophical society 19 



server could always be considered alike. Mr. Coblentz stated there 

 was no great reason for testing the possible difference between the 

 eyes of the observer. Mr. Swann referred to the limit of sensitivity 

 of the human eye. Mr. Wenner noted that the visibility curve, as 

 measured, depends on many things. What difference would there be 

 by using equal-energy spectrum or equal light throughout spectrum? 

 Mr. Coblentz thought that any difference would depend upon the 

 stimulus started with. Mr. Taylor stated that it was very interesting 

 to note the efficiency of ordinary illuminants. Mr. Wells referred 

 to the practical application to photometry. 



Informal communications. Mr. W. F. G. Swann presented an in- 

 formal communication, On the absorption of energy by an electron. In 

 order that an electron may absorb an appreciable amount of energy 

 from a wave falling upon it, it is frequently considered that the energy 

 of the wave must be concentrated in a filamentary manner, the idea 

 being that the electron can only take energy from a cross section of 

 the advancing wave comparable with the cross section of the electron. 

 That such an assumption is necessary has never appeared very con- 

 clusive to the author, and the object of this work is to point out that 

 if the difficulty of an electron's absorbing energy from a cross section of 

 the wave, large compared with its own cross section, be admitted, we 

 must confront this same difficulty in problems of much simpler nature 

 than those which are generally quoted as the problems which give 

 rise to the difficulty. Thus, suppose an electron to be placed in a uni- 

 form field X of magnitude 1 volt per cm. The work done on it by 

 the time it has moved a distance x is Xex. If we calculate the volume 

 W of the original field which would contain energy equal to this amount, 



X 2 

 remembering that the energy density is ^ — - v we readily find W = 



-^= — . Thus by the time the electron has moved 0.05 cm. in a field 

 X J 



of 1 volt per cm., it will have absorbed energy equivalent to that con- 

 tained in a volume of the original field equal to 18 X 10~ 8 cc, or in 

 a sphere whose radius is about 3 X 10 -3 cm. or 3 X 10'° times the ra- 

 dius of an electron. 



The 778th meeting was held on November 11, 1916, at the Cosmos 

 Club. President Briggs in the chair; 42 persons present. The min- 

 utes of the 777th meeting were read in abstract and approved. 



Mr. G. K. Burgess presented a communication, illustrated with 

 lantern slides, on The resistivity and thermoelectric properties of pure 

 iron. There were described the apparatus and experimental methods 

 used and results obtained in exact measurements of the resistance and 

 the true thermoelectric power of pure iron over the temperature range 

 0° to 1000°C. Both series of measurements were taken in vacuo at 

 2° intervals using iron 99.968 pure. The characteristics of the trans- 

 formations A 2 at 768° and A z at about 910°C. and the lack of other 



