1041 



The mechanical equivalent of light. 



According to our results the brightness of the black body at 1336'^ 

 K would be 0,119 ^^Vcm'.'rhe light-tlux is therefore 0,119 ji /y''""^"/c„v 

 But this flux is also equal to: 



. , , =;r0,119 



— [e, V) dl 







where M is the mechanical equivalent of light i. e. the number of 

 watts required to obtain a light-flux of 1 //lumen of the wave-length 

 for which the sensitiveness of the eye for light is a maximum; 

 E). being the intensity of radiation of the black bod} in the wave- 

 length X at 1336°, F/ the sensitiveness of the eye for light of wave- 

 length ).. 



Calculating M for the determinations of the luminosity curve of a 

 number of observers we found using the measurements of 



CoBLENTz and Emerson (125 persons)') 3/= 0,00154 ^«"Vh. Lumen. 



Ives and Kingsbury (61 persons)") 0,00151 



Nutting (21 persons)') 0,00135. 



The constants in Planck's formula^) were taken as follows: 



C, = 3,704 10-12 Watts/^^,^^, 



c, =: 1,4300 genomen. 



With 1 watt a mean spherical intensity of 51.7 H-Oandles may 

 thus be obtained. 



We add the results of a few more observers. 



BuissoN and Fabry*) J7 .:= 0,00J30 ^/hl 



Nutting^) (acetylene flame) 3/:= 0,00108 



Langmuir '') (tungsten-lamp luminosity curve 

 according to Nutting) M = 0,00109 



Ives and Kingsbury*) (various methods) J/ ^0,00143 



PiRANi and MiETHiNG ') (measurement on lamps) J/ =i 0,00123 



1) CoBLENTZ and Emerson. Ref. El. World (69) 1117, 1917. 



2) Ives and Kingsbury. Phys. Rev. (6) 319, 1915. 



3) Nutting. Phil. Mag. (29) 301, 1915. 



^) Co was taken in accordance with the new temperature scale of the Ph. T. R. 

 This choice further involves /i = 6,55 10— 27, N=&,0b[0is, cr = 5.75 10-12 

 ^^"Vcm" in good agreement with the most recent measurements. 



6) BuissoN and Fabry. Compt. Rend. (153) 254, 1911. Gomp. also Ives, Goblentz 

 and Kingsbury. Phys. Rev. (5) 269, 1915. 



«) Nutting. Phil. Mag. (29) 301, 1915. 



7) Langmuir. Phys. Rev. (7) 302, 1916. 



8) Ives and Kingsbury. Phys. Rev. 1915 and 1916, various articles. Final value 

 in (8) 254, 1916. 



») PiRANi and Miething. Verh. D. Phys. Ges. 219, 1915. 



