158 Profs. Hagen and Rubens on some Relations 
- In a former paper* we have shown that some of these con- 
tradictions begin to disappear if, instead of examining the 
ultra-violet or the visible spectrum, we advance to longer 
wave-leneths. We found that platinum, which, in the visible 
and ultra-violet spectrum, is much more opaque than gold 
and silver, becomes more transparent than these metals in 
the infra-red. We have lately stated that the same is true 
in a higher degree with respect to bismuth. A thin layer of 
bismuth of about 90 wy thickness, which scarcely transmits 
1/1000 part in the red, possesses a transparency of 10 per 
cent. at a wave-length not longer than \=4 uw. Consequently 
it did not seem improbable that, passing to still greater 
wave-lengths, values in accordance with Maxwell’s theory 
would be obtained for the diathermancy of metals. Supposing 
Maxwell’s theory to be correct, this would mean that the 
influence of the molecular periods of the different metals 
vanishes gradually in the infra-red region, with increasing 
wave-lengths. 
Relations quite similar to those that Maxwell’s theory 
demands for the transparency of metals, can be foreseen for 
the intensity of the radiation penetrating into the metals, and 
. for the power of emission. But these values are much easier 
to determine than the transparencies. In the first place, in- 
vestigations of the transparency require considerably greater 
intensities of radiation. Secondly, no substances exist which— 
in that part of the spectrum—are sufficiently transparent for 
heat-rays, and can at the same time serve as supporters of 
such thin layers of metalT. Lastly, the number of metals 
suited to the examination of transparencies is much more 
limited than that suited to measurements of the emission or 
reflecting-power. This is owing to the fact that the con- 
struction of good reflecting-mirrors is much easier than 
that of metallic layers of equal thickness and perceptible 
transparency. 
The intensity of radiation entering into the metals can be 
measured in different ways. The simplest method is by 
determining the reflecting-power, or by measuring the power 
-of emission. 
If R represents the reflecting-power of a metal for normal 
incidence expressed in percents of the incident radiation, 
the intensity of the radiation entering the metal is 
I=(100—R). Inthose parts of the spectrum in which the 
* K. Hagen & H. Rubens, Ann. d. Phys. viii. p. 482 (1902). 
t+ Rocksalt, sylvine, and silver chloride, which possess a sufficing 
transparency, cannot be used in consequence of their unfavourable chemical 
qualities, : 

