174 Profs. Hagen and Rubens on some Relations 
corresponding change of the emission-power for long waves*. 
Otherwise the constant C=(100—R)V/ K;79 for pure metals 
would have been found 25 per cent. smaller than that for the 
alloys with exceedingly small temperature-coefficients, and 
could agree only by a mere chance with the theoretical value 
C23. 
Our experiments on emission therefore enable us to cal- 
culate the change of resistance with the temperature, since 
the emission increases with the square root of the resistance. 
The following experiments have been undertaken in order to 
obtain a still stronger proof of this relation. A hollow case f 
of platinum-foil, heated by an electric current, was placed in 
front of the diaphragm C,. The temperature of the case 
could be measured by aid of a thermo-element LeChatelier, 
the welded end of which was placed inside. For higher 
temperatures (800°-1560°) the hollow case was replaced by a 
single platinum strip, cut from the same piece of platinum- 
foil of which the case had been made. The temperature of 
the incandescent platinum strip was determined by means of 
the optical pyrometer, lately described by Holborn and 
Kuribaum {. The “black temperature ” observed herewith 
was transformed into Celsius degrees, according to the table 
of correction given by these authors. For the experiments 
with higher temperatures (above 800°) we were obliged to 
increase the number of the reflecting fluorite surfaces from 
3 to 4; otherwise the ‘* Reststrahlen ” would not have been 
sufficiently pure. The still remaining small impurities, caused 
by radiation of short wave-length, were determined by the 
interposition of a plate of rock-salt, and the amount deducted. 
For the high temperatures we used the diaphragm C,, only 
14 mms. wide, thus permitting of direct comparison between 
the radiation of the hot platinum-foil and that of our “ black 
body ” at 170° Celsius. But when the intensity of radiation 
of the black body for “residual rays” of fluorite is known 
for one temperature, it is easily determined for every other § ; 
since the intensity of the observed “ residual rays ’’—for 
* In the visible spectrum the change of the optical constants of the 
metals with the temperature is exceedingly small, as is shown by the 
observations of Messrs. R. Sissingh (Arch. Néerland. xx. p. 172, 1886), 
P. Drude (Wied. Ann. xxxix. p. 538, 1890), B. Zeemann (Commun. of 
the Lab. of Physics at the Univers. of Leyden, No. 20, 1895), and 
A. Pfliiger (Wied. Ann. lvii. p. 498, 1896). 
+ A hollow case of this kind was first employed by Messrs. O. Lummer 
_and F. Kurlbaum, Verhandl. d. Phys. Gesellsch. zu Berlin, xvii. p. 106 
(1898). 
t L. Holborn & F. Kurlbaum, Ann. d. Phys. x. p. 225 (1908). 
§ H. Rubens & F. Kurlbaum, Ann. d. Phys. iv. p. 649 (1901). 

