between Optical and Electrical Qualities of Metals. 1959 
energy suffices for exact measurements, and R is sufficiently 
different from 100, the determination of I can be successfully 
performed by measuring the reflecting-power. But itis just for 
the greatest attainable wave e-lengths that these conditions 
are not sufficiently fulfilled. Ther refore, i in these regions it is 
preferable to determine I by measurements of the power of 
emission. If E be the emission of an opaque polished metal sur- 
face, and e that of “a black body” of the same temperature— 
both for the same wave-length—we obtain I[=H/e according 
to Kirchhoff’s law. 
Consequently our experimental research consists of two 
parts. In the first I is determined with the aid of the 
reflecting-power, in the second by measurement of the 
power of emission. 
le 
The Rejlecting-Power of Metals in the Infra-Red. 
The reflecting-power of metals has often been the object of 
experimental research. In a paper published in 1889 by one 
of us*, the reflecting-power of a series of metals has been 
examined. From tbe results the conclusion was drawn that 
the better conductors of heat and electricity (silver, copper, 
gold) show a higher reflecting-power for infra-red rays than 
do the other metals. In addition to this paper, and that of 
A. Trowbridget, our own recent research must be mentioned. 
But owing to the imperfection of the material and the surface 
of the mirrors employed, the results of the older researches do 
not permit any definite comparison with the electromagnetic 
theory. On the other hand, our own measurements t did not 
pass beyond the limit of 15 #, a wave-length not sufficing for 
this purpose. 
Method of Observation. 
The arrangement of our apparatus is given infig. 1. H 
designates the slit of a spectrometer, furnished with silver 
mirrors (K and L) instead of lenses, in such way that H K 
represents the collimator, and L T the observing telescope. 
T is a linear thermopile, described in an earlier paper by one 
of us§. The prism P was of fluorite for the wave-lengths 
* H. Rubens, Wied. Ann. xxxvii. p. 249 (1889). 
+ A. Trowbridge, Wied. Ann. Ixy. p. 595 (1898). The papers of 
S. P. Langley, Phil. "Mag. xxvii. p. 10 (1889), E. kr. Nichols, Wied. Ann. 
lx. p. 401 (1897), and F. Paschen, Ann. d. Phys. iv. p. 304 (1901) con- 
tain only measurements of the reflecting-power of silver in the infra-red. 
.{ E. Hagen & H. Rubens, Ann. d. Phys. viii. p. 1 (1902). 
§ H. Rubens, Zeitschrift fiir Instrrumentenkunde, xvii. p. 65 (1898). 
