between Optical and Electrical Qualities of Metals. 179 
theory leads 
equation T 

as previously shown by Mr. Drude *—to the 
Gaay aio aay te ee {0) 
where g is the coefficient of extinction, v the index of refrac- 
tion of the metals for normal incidence. This equation, 
which is only approximately correct, shows that the index 
of refraction and the extinction coefficient are numerically 
equal for long waves. 
Besides 
R=100(1—*)=100(1— a 
therefore 
200 
100—R* 
Consequently both values are definable from the emission- 
power alone. 
7. A further consequence, resulting from the agreement 
of our researches with the electromagnetic theory of light, 
deserves special mention. Besides abstract numbers, the 
theoretical computation of the constant C contains only the 
velocity of light and the wave-length, both of which can be 
determined by experiments on radiation. By dividing the 
emission-power of a metal for the wave-length A» (the 
emission of the black body being rated at 100) by the con- 
stant C, and by squaring the ratio, we obtain the electrical 
resistance in ohms of a wire of the respective metal (1 m. 
leneth and 1 mm.’ cross-section). So it is now possible to 
undertake absolute determinations of electrical resistances 
solely by the aid of measurements on radiation. 
jr=Vv= 
* P. Drude, Physik des Aethers, p. 575 formula (68), 1894; and 
M. Planck, /.c. In the footnote p. 166 of this paper we have given 
Planck’s enunciation of formula (6). 
+ It follows from formula (6) that the extinction-coefficient (g) 
increases, for long waves, with the square-root of the wave-length. The 
absorption-coefficient a,= = which characterizes the real absorption 
of the metals, consequently diminishes proportionally to the square-root 
of the wave-length. Nevertheless the absorption of the metals remains 
very considerable, even for waves longer than one metre. A metal 
layer of about z3> mm. thickness must necessarily absorb the whole 
infra-red spectrum. Itis therefore impossible that intra-red rays of great 
wave-length pass through layers of aluminium half a millimetre thick, 
and it follows that M. Blondlot’s so-called “‘ Rayons N ” cannot possibly 
be infra-red rays. 
N2 
