39° 



00' 



38 



50 



38 



40 



38 



30 



38 



20 



38 



10 



38 



00 



and Unmeasured Wave-lengths. 401 



The " transmission " of the plate of rock-salt (here uncor- 

 rected for heat lost by reflection at the anterior surface) 

 appears to be shown by subsequent experiments to slightly 

 diminish for extreme heat-rays in the Leslie cube-spectrum ; 

 but to remain so nearly constant through the range of these 

 experiments, as to show that the present approximate values 

 need no correction on this account. More exact ones decisive 

 of the point in question will be given in a subsequent 

 memoir. 



The folio win o- series was then taken for fixing more 

 accurately the position of the maximum :-— 



Deflection. 



365 

 406 

 430 

 428 

 414 

 401 

 371 



From curves representing these observations, it was con- 

 cluded that the maximum was at 38° 35'. It will be shown 

 further on, how an attempt may be made to estimate the 

 wave-lengths in these regions. 



Measurements were also made with surfaces of copper 

 heated to much higher temperatures, and with the cube at 

 different lower temperatures, for the purpose of determining 

 whether the position of the maximum of the energy curve 

 varies with the temperature, and if so to determine if possible 

 the relation. Experiments of this kind have been made by 

 Mr. W. W. Jacques *, who found that " the distribution of 

 heat in the spectrum of a solid or liquid source of radiation is 

 nearly independent of the temperature of the source."" It 

 was evident from the care with which Mr. Jacques's experi- 

 ments were conducted, that the shifting of the maximum 

 must be slight and difficult of quantitative determination; 

 but with the pure spectrum and delicate heat-measuring 

 apparatus at our command, it was thought possible that this 

 might be effected. Accordingly, measurements similar to 

 those just described were made with a radiating surface of 

 lampblacked copper at the approximate temperatures of 815° 

 C, 525° C, and 330° C. ; and with the Leslie cube at tem- 

 peratures of 178° C, 100° C, 40° C, and -20° C. ; the 

 excess over the temperature of the room being in the latter 



* " Distribution of Heat in the Spectra of various sources of Radia- 

 tion," by William W. Jacques, Ph.D., Proceedings of American Academy. 

 Phil. Mag. 8. 5. Vol. 21. No. 132 May 188C 2 F 



