Mechanics of Luminosity. 259 



acetate lamp expressed in terms of that of the glowing 

 platinum for the yellow 



E x =0-24. 0-547 E = 0*13 E. 



Comparison of Sodium Flame and Glowing Platinum. 



24. After this determination we may further compare the 

 brightness of the amyl acetate lamp for the yellow with that 

 of a gas-flame coloured yellow by sodium, according to the 

 method of Herr Ebert*, and thus the latter also with the 

 brightness of the yellow of glowing platinum. 



If, then, we wish to determine the ratio of the radiation of 

 the sodium flame corresponding only to the yellow sodium 

 lines and the total energy of radiation of the glowing plati- 

 num, we must first determine the ratio of the latter to the 

 radiation which reaches a definite portion of the yellow. 



For this purpose we will make use of the results of 

 Moutonf, by assuming, without doubt correctly, that the 

 temperature of the platinum wire in our absolute measure- 

 ments is nearly equal to that of the platinum wire in 

 Mouton's Bourbouze lamp. 



If this is not exactly the case, and consequently the final 

 value is not quite accurate, yet its order of magnitude can in 

 no case be affected. 



In order to obtain a part of the radiated energy which 

 belongs to a definite portion of the spectrum situated in the 

 neighbourhood of the D-line, the following method was 

 adopted. A curve was drawn upon paper according to 

 Mouton's numbers, which represented the distribution of 

 energy as a function of the wave-length. The wave-lengths 

 were measured in 1/jl, the energies in any convenient unit 

 By division of the weight g of a piece of the curve-paper of 

 known area by the weight G of the area included between 

 the curve and the axis of abscissae, we obtain for the fraction of 

 the total energy corresponding to unit area 



* Wied. Ann. xxxii. p. 345 (1887). 



t Compt. Rend, lxxxix. p. 295, 1879 ; Beibl. iii. p. 868, 1879. The 

 following calculation of course proceeds upon the assumption that we 

 obtain the whole quantity of radiated energy in the bolometer or the 

 thermopile, or that the substance of the bolometer absorbs even the extreme 

 infra-red rays. This may be tested experimentally by comparing the curve 

 of energy determined by the bolometer with the total expenditure of 

 energy as measured by resistance and intensity. I should have liked to 

 have determined the distribution of energy for the wire employed by me, 

 but unfortunately this was not possible with the very unfavourable con- 

 ditions of the Erlanger Institute — it is so exceptionally damp that it is 

 not possible to set up rock-salt prisms &c. for the purposes of an extended 

 research. 



