114 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Beckman at wave length 24 was found to be 24,250. Lumraer and 



Pringsheim find, moreover, that the logarithmic isochroms, especially 



when extended to higher temperatures, are not straight lines, but show a 



, . 1 

 slight convexity towards the — axis. 



Exception has also been taken to the Wien-Planck formula on the 

 ground that it gives for infinite temperatures a finite limit to the value of 

 the intensity, a result which Rayleigh * in a recent paper has character- 

 ized as physically improbable. 



Rayleigh proposes the form 



. — 4 — 



ft 



2 



I= Cl T\~ % e~^T 



but Lummer and Pringsheim find that this likewise fails to properly express 

 their experimental results. Lummer and Jahnke propose, in view of 

 these discrepancies, to give the equation the general form 



/= CT 5 (XT)-* e-(^) v ' 



an expression which coincides with Wien's formula for ft = 5 and with 

 Rayleigh's for [i = 4. They find the measurements of Lummer and 

 Pringsheim satisfied when p lies between 4.5 and 5, and v lies between 

 .9 and 1.0. If we accept the value /< = 5 and v — 0.9, this equation 

 always leads to a finite value of intensity for infinite temperature. All 

 other values of these quantities give infinity as the limit of intensity. 



Whether logarithmic isochroms or the value of the quantity c 2 , computed 

 from measurements upon carbon rods, would aid in deciding between the 

 various equations under discussion is a question. The data given in this 

 paper would not lead us to class the carbon rods studied as black bodies. 

 The emissive power of various forms of carbon is well-known to be 

 smaller than that of the ideal black body, and there is no reason to 

 suppose that it is independent of the temperature. The relative lagging 

 behind of the intensities in the red might perhaps be taken a3 an indica- 

 tion of a tendency to approach the infinite maximum demanded by the 

 Wien-Planck formula ; but the isochrom for .76 shows that the effect, if 

 it exists, must be looked for at some much higher temperature than that 

 covered by these measurements. In spite of these doubts as to the 

 applicability of the measurements on carbon rods to the problem of the 



* Philosophical Mag., XLIX. 539 (1900). 



