326 Profs. H. du Bois and H. Rubens on Polarization of 



II. Experimental Arrangement. 



§ 5. Our present investigation comprises essentially the 

 wave-length region from 24-108 //- ; we have carried out 

 our measurements for the fluorite and rocksalt Reststrahlen 

 as well as for the long-waved rays of the Welsbach-mantle, 

 isolated by means of quartz lenses*. In addition we have 

 made measurements on the exceptionally long-waved rays of 

 mean wave-length 314 /jl, produced by a quartz-mercury 

 lampf. On account of the slight intensity, however, we 

 could in this case investigate the grating only in its normal 

 position. All these rays are by no means homogeneous ; their 

 energy distribution is, however, sufficiently well known, and 

 their non-homogeneity in general plays no very great part. 



The Reststrahlen of fluorite, if they are produced by a 

 Welsbach mantle and separated out by reflexion at three 

 fluorite surfaces, possess an average wave-length of 25*5 /n. 

 The homogeneity is greatly increased by the introduction of 

 a thin plate of sylvine in the path of the rays. The energy 

 curve approaches symmetry and the mean wave-length is 

 decreased to about 24 fi. We have availed ourselves of this 

 simple resource by introducing a plate of sylvine of 1*75 mm. 

 thickness in the path of the rays, which served as a window 

 for the air-tight bell of our radiomicrometer. 



New experiments have shown that the Reststrahlen of 

 rocksalt, as produced by the method here applied, consist of 

 two bands, of which the first is situated at 46' 9 /x, the other at 

 53' (5 fi J ; the mean wave-length is about 52 /a. It is supposed 

 in this case that the rays are transmitted through a quartz 

 plate 1'2 mm. in thickness. This condition was approximately 

 fulfilled, for the radiomicrometer window in our investigation 

 consisted of a quartz plate 1 mm. thick. 



The energy of radiation in the case of the long-waved 

 heat rays of 108 /a isolated from the Welsbach mantle by 

 means of quartz lenses was barely sufficient for measurements 

 to be carried out with gratings normal to the path of the rays ; 

 in inclined positions, on the other hand, the thickness of the 

 quartz layer within the path of rays had to be reduced from 



* H. Rubens and R. W. Wood, Berl. Sitzungsber. p. 1122 (1910). 



t H. Rubens and O. v. Baeyer, B*rl. Sitzungsber. p. 339 (1911); 

 Phil. Mag. [6] xxi. p. 689 (-191 1 ) ; Verh. D. Phys. Ges. xiii. p. 393 (1911). 

 This non-homogeneous radiation, separated out by means of quartz 

 lenses and filtered through black cardboard, consists principally of two 

 parts, the mean wave-lengths of which are 218 ju and 343 jx respectively, 

 of such a kind that the intensity of the long-waved portion is three times 

 as much as that of the short-waved. 



X H. Rubens and H. Hollnagel, Berl. Sitzungsber. 1910, p. 26. 



