170 Profs. Hagen and Rubens on some Relations 
black body for long waves, a process causing no difficulty 
when sufficient energy is at hand *. 
Fig. 3 shows the arrangement of apparatus which served 

for these experiments. A is a copper box that could be 
turned at will: its four sides contained round openings pro- 
vided with screws, into which the metal plates t (radiating 
* A similar method has been tried by O. Wiedeburg (Wied. Ann. 
lxvi. p. 92, 1898) to compare the total emission of different metals with 
that of silver. The results which he obtained support those already 
published by H. Rubens (Wied. Ann. xxxvii. p. 249, 1889), that the 
good conductors for heat and electricity show a higher reflecting-power 
than the other metals. But, in view of our measurements of the reflect- 
ing-power, Wiedeburg’s numerical data of the emission-power seemed so 
improbable, that we repeated his experiments for several metals. Doubt- 
less his experiments are influenced in a very high degree by diffused 
foreign radiation, a fact which is proved by the following table. Here we 
give the emission of four metals at 100°, silver being the unit, in the first 
place according to O. Wiedeburg’s, and secondly according to our own 
observations. The latter cannot possibly be influenced by any foreign 
radiation. 
Total emission at 100°. 
Observers. Silver. Platinum. Steel. §Manganin. 
O: Wiedeburg .2 22% 1:00 1:23 1:31 1:32 
Hagen and Rubens. . 1:00 4°65 6°66 8:00 
+ It is a special advantage of this method that the metal plates used 
need only to be pure and well polished, but not plane. 

