52 Messrs. Paterson and Dudding on Estimation of 



extent of the work described here does not justify the 

 wider application of equation (5) to the whole spectrum 

 for substances which do not behave as true black or grey 

 bodies. 



Lummer and Pringsheim*, investigating platinum, state 

 that the distribution of power throughout the whole spectrum 

 for platinum is given by the following equation : — 



15,600 



E x = a\~ 6 e~ AT - , (10) 



corresponding to the form for an ideal black body of 



1 4,500 



Ea = C 1 \-%" at (11) 



If this assumption were justifiable, the authors' values for 

 tungsten work out very nearly the same as those given by 

 Lummer and Pringsheim for platinum; but, for the reasons 

 just stated, too much significance must not be attached to 

 this agreement. 



Coblentz t expresses the opinion, based on an investigation 

 of several metals, that " n " in equation (5) is not a constant, 

 hut is a function of wave-length and temperature. 



There is nothing in the results discussed above which is in- 

 consistent with either Lummer and Pringsheim's or Coblentz's 

 suggestions. 



Throughout this work the practical case has been dealt 

 with of filaments mounted in exhausted globes in which there 

 is undoubtedly some loss of watts and efficiency due to the 

 cooling- effect of the leading-m wires. If conclusions of a 

 fundamental nature are to be drawn from the results obtained, 

 it is necessary to know to what extent this cooling is likely to 

 affect the constants given in equations (8) and (9). 



In the work already referred to by Hyde and Cady, figures 

 are given for the loss of watts by conduction at the ends of 

 filaments, and columns I. to IV. of the following table are 

 from this paper. 



In columns Y. and YI. are tabulated the values of lumens 

 per watt calculated from column II., using average values for 

 the reduction factors of the types of lamp under consideration, 

 and the corresponding temperatures taken from the curves in 

 fig. 3. 



* Lummer and Pringsheim, Verhandlungen der Deulschen Phys. Gesell. 

 pp. 23-25 (1899). 



t Bulletin B. S. vol. v. pp. 388-379 (1908-1909). 



