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LXXIX. Measurements in the Extreme Infra-Bed Spectrum. 

 By H. Eubens and H. Hollnagel *. 



[Plate XIII.] 



FOR the investigation o£ the extreme infra-red portion 

 of the spectrum the method of " Reststrahlen " has 

 proved most fruitful. Formerly the diffraction grating has 

 been utilized to determine the wave-length of any long- 

 waved radiation-complex, isolated by means of selective re- 

 flexion, as, for example, the " Reststrahlen " of rock-salt 

 and sylvine ; interference methods have lately been applied 

 to the same end for " Reststrahlen " of shorter wave- 

 length f. Such methods suitably modified are also adaptable 

 to a study of the composite spectral structure of radiations 

 of greater wave-length. To be sure there are exceptional 

 difficulties to be overcome which consist principally in the 

 fact that no solid substance is known at present which in 

 comparatively thick layers is completely transparent below 

 45 /*, as rock-salt for the radiation of 10 /x wave-length for 

 example. An interferometer which will enable one to 

 measure the longer-wave radiations must satisfy these con- 

 ditions : the substance traversed by the rays to be investi- 

 gated must be in plates of a minimal and invariable thickness 

 and the radiant beam must be diaphragmed as little as possible. 

 In obviating such a diminution of energy, the interference 

 method has a decided advantage over all spectrometric 

 methods, where the use of a slit is unavoidable. A second 

 advantage consists in the evasion of the diffraction grating 

 which is so uneconomical as to energy. 



By the adaptation of such an interference method we have 

 succeeded not only in attaining a much higher precision for 

 wave-length measurements, but have also been able to 

 penetrate to a region of much longer waves. 



The Interferometer. 



The principal part of our interference apparatus con- 

 sisted of an air-film, rendered plane-parallel by two quartz 

 plates ; the thickness of this film could be continuously 

 varied and measured. The construction of the interfero- 



* Communicated by the Authors. 



t John Koch, Ann. d. Phys. xvii. p. 658 (1905) ; Nova Acta Regice 

 Societatis Scientiarum Upsalensis, ser. iv. vol. xi. no. 5, 1909. 



