NICHOLS. — THE VISIBLE RADIATION FROM CARBON. 113 



radiation as a function of wave length and temperature. The equation 

 reached from quite different points of view by Wien * and by Planck, f 



/= Cl A -5 e-Ar' 



in particular, has been the subject of exhaustive discussion and of 

 experimental tests. To this end Paschen $ determined with the bolo- 

 meter the distribution of energy in the infra-red spectra of various 

 bodies from 15° C to 1300°. The materials thus subjected to measure- 

 ment were oxide of copper, platinum, lampblack, and graphitic carbon. 

 The range of wave lengths explored extended from 9.2/t to 0.7 ( «. 

 Luiumer and Pringsheim § made similar determinations upon the ideal 

 black hotly, and Lummer and Jahnke || finally repeated these measure- 

 ments in the case of the black body and of platinum. Wanner,! working 

 with Paschen, made careful spectrophotometric measurements of the 

 visible radiation from the ideal black body. To test the applicability of 

 the Wien-Planck formula to these measurements, the equation is given 

 the form, — 



log 7= -vi — y» y> 



in which 



yi = log (<?! A -5 ), 



72 = ^ log e. 



The isochromatic curves are then plotted with the logarithm of the 

 intensities as ordinates and the reciprocal of the absolute temperature as 

 abscissae. The agreement of the equation with the observations is 

 found in the fact that isochroms thus plotted, at least as far as the work 

 of Paschen and Wanner is concerned, always take the form of straight 

 lines, and that the quantity r 2 computed for various wave lengths is 

 found to be a constant. Lummer and Pringsheim, on the contrary, find 

 in the discussion of their measurements that the constant, c 2 increases 

 steadily with the wave length from 13,500 at 1.2 p to 16,500 at 5 p, and 

 18,500 at 0.3 p. The value of c 2 computed by measurements from 



* Wien, Wiedemann's Annalen, LVIII. 662 (1896). 



t Planck, Drude's Annalen, I. 69 (1900). 



t Paschen, Wiedemann's Annalen, LVIII. 455 (1896); also LX. 662 (1897). 



§ Lummer and Pring-sheim, Deutsche phys. Gesellsehaft, I. 23, II. 16o (1900). 



|| Lummer and Jahnke, Drude's Annalen, III. 283 (1900). 



1 Wanner, Drude's Annalen, II. 141 (1900). 



VOL. XXXVII. — 8 



