S. P. Langley — Observations 



on Invisil 



Deviation. 



Deflection. 



39° 00' 



375 



38 50 



406 



38 40 



430 



38 30 



428 



38 20 



414 



38 10 



401 



38 00 



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 inde- 

 pendent of the temperature of the source." It was evident 

 from the care with which Mr. Jacques's experiments were con- 

 ducted, that the shifting of the maximum must be slight and 

 difficult of quantitative determination, but with the pure spec- 

 trum and delicate heat-measuring apparatus at our command, 

 it was thought possible that this might be effected. Accord- 

 ingly, measurements similar to those just described were made 

 with a radiating surface of lamp-blacked copper at the approxi- 

 mate temperatures of 815° C, 525° C. and 330° 0., and with 

 the Leslie cube at temperatures of 178° C, 100° 0., 40° C, and 

 — 20° C, the excess over the temperature of the room being in 

 the latter cases respectively 185° C, 88° C, 46° C, and -18° 

 0. In the last instance, the cube was colder than the bolom- 

 eter strips, and the deflections obtained were negative; though 

 small, they were distinctly measurable, the greatest being —12 

 divisions of the galvanometer scale, f We have in Plate 3 the 

 curves representing the radiation from these sources, in which 

 the abscissae are proportional to the indices of refraction in the 

 rock-salt prism, but the ordinates only approximately so to the 

 deflections of the galvanometer due to the heat at the corre- 

 sponding points, since we are not here primarily concerned with 

 the relation of the amounts of heat emitted to the temperatures 



* Distribution of heat in the spectra of various sources of radiation, by William 

 W. Jacques, Ph.D., Proceedings of the American Academy. 



f The position of the maximum in the ice curve is indicated, but the curve itself 

 is, on this scale of ordinates, sensibly coincident with the straight line. (See 

 Plate 1.) 



