Physics. — ''The brightness of the black body and the mecha- 

 nical equivalent of light." By. Dr G. Holst and J.Scharpde 

 Visser. (Communicated by Prof. H. Kamehlingh Onnes.) 



(Communicated in the meeting of Sept. 29, 1917). 



Introduction. For various investigations it is of great importance 

 to know the brightness of the black body at various tempera- 

 tures. Direct measurements were made by Lummer and Pringsheim') 

 and by Nernst '). Nernst sums up his results in tlie formula 



11230 

 logB=b,3Q7 ^- {H= intensity of 1 cm' in Hefner-candles, 



7'= absolute temperature). The curve corresponding to this formula 

 passes closely through Lummer and Pringsheim's points. Recently, 

 however, doubts have arisen as to these measurements. Various 

 investigators starting from the spectral distribution of energy, the 

 sensitiveness of the eye and the mechanical equivalent of ligiit have 

 calculated the brightness of the black body and all of tliem come 

 to much higher values than were found experimentally. The following 

 table shows the results: 



1) Lummer and Pringsheim. Pliys. Zeitschr. (2) 97, 1901. 



2) Nernst. Phys. Zeitschr. (7) 380, 1906. 



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



*) PiRANi and MiETHiNG. Verh. D. Phys. Ges. 219, 1915. 



6) Ives and Kingsbury. Phys. Rev. (8) 323 1916 and mech. equiv. ilf= 0.00159 

 Phys. Rev. (8) 254, 1916. 



6) Foote and Fairchild. Scienl. papers Bur. Stand, n». 270 and ilf = 0.00159. 



