112 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The work upon treated carbons was confined chiefly to high tempera- 

 tures, a sufficient number of readings within tlie range already explored 

 with the untreated carbons being taken to show that the distribution of 

 intensities at the lower temperatures did not differ materially from that 

 in the spectrum of the former. The set of isotherms given in Figure 22 

 will suffice to indicate the general character of the re^ults. It will be 

 seen that in this case, as in that of the untreated carbon, the concavity of 

 the curve between .Q/j and the red end of the spectrum is well marked 

 at 13G5° ; and that at 1515° there was a well-pronounced maximum at 

 about .65,M. The greater stability of the treated carbon made it possible 

 to obtain consistent measurements on a number of rods at temperatures 

 above 1500° and to establish beyond doubt the t^orm of the curves. It 

 is obvious that for the study of the spectrum of incandescent carbon at 

 this and higher temperatures the conditions would be much more 

 favorable in the case of the incandescent lamp than with rods mounted 

 in a large vacuum chamber like tliat used iu tlie present investigation. 

 Lamp filaments in the process of manufacture are brought by thorough 

 carbonization into a condition to withstand permanently much higher 

 temperatures than the rods at my disposal were capable of doing. 

 There is as yet, it is true, no direct means of determining the tempera- 

 ture of the lamp filament ; but the curve for the relation of electromotive 

 force to temperature (Fignre 11) is of such a character as to lead us to 

 expect that comparisons of the spectra of incandescent lamps, in which 

 electromotive forces were used as a criterion of the deajree of incan- 

 descence, would at least enable us to confirm the existence of the 

 remarkable phenomenon brouglit out by the present experiments and to 

 extend observations of it to still higher temperatures. 



Mr. Ernest Blaker has, since the completion of the measurements 

 described in this paper, compared the visible spectrum of lamps with 

 treated fihiments, and of lamps the filaments of which before exhaustion 

 had been coated with lampblack, with the spectrum of the acetylene 

 flame. His measurements confirm very completely those which I have 

 described in this paper, and contribute important evidence in favor of the 

 existence of this anomaly in the law of distribution of intensities in the 

 spectrum of glowing carbon. 



Theoretical Aspects of the Foregoing Data. 



The efforts of stuflents of radiation have of late yeai's been directed 

 particularly to the testing of the various formulae by means of which 

 the mathematical physicists have attempted to express the intensity of 



