Rubens and Trowbridge — Rock Salt and Sylvine, etc. 33 



Art. Y. — On the Dispersion and Absorption of Infra- 

 Red Rays in Rock Salt and Sylvine /* by H. Rubens and 

 A. Trowbridge. 



A short time ago it was pointed out that it is possible to 

 isolate a narrow band of approximately homogeneous rays 

 from the great number sent out by a source of heat by means 

 of successive reflections from a number of surfaces of the same 

 substance. This isolation follows from the selective reflection 

 of the surfaces used.f In this manner comparatively mono- 

 chromatic rays, in some cases possessing a very great mean 

 wave length, can be obtained without the use of prism or grat- 

 Compared with the usual method, where dispersion is 

 obtained by means of a prism or grating, this method has, for 

 some research work, the disadvantage that the wave length 

 cannot be varied at will; as one is limited to well defined 

 wave lengths determined by the substance of the reflecting 

 surfaces. For this reason it is of some interest to state to 

 what extent the prismatic method is applicable in the further 

 infra-red spectrum ; or else, what wave length can be attained 

 in a spectrum thrown by a sylvine or rock-salt prism. 



There are several properties of a body upon which its fit- 

 ness as a substance for prisms depends. Next in importance 

 to its occurrence in large clear pieces and the possibility of 

 preparing and preserving plane surfaces, come its dispersion 

 and absorption. We undertook, therefore, to determine with 

 the greatest possible precision, the dispersion and absorption of 

 these bodies for infra red rays of great wave length. 



Measurement of Dispersion. 



The dispersion of rock salt and sylvine has been compared 

 with that of fluorite as far as the wave lengths 8'86/jl and 7'06/jl 

 respectively, in a previous investigation undertaken by one of 

 us4 For fluorite there exist the careful measurements of its 

 dispersion by Paschen,§ which were made with the help of a 

 Rowland grating especially constructed for this purpose. 

 These measurements extend to the wave length X = 9*44/x and 

 were made after Langley's method. Several additional points 

 of the dispersion curve of rock salt and sylvine have lately 

 been observed in the extreme infra-red at X = 20*6/x and X = 



* Translated by one of the authors from the original in Wiedemann's Annalen. 

 f H. Rubens and R. P. Nichols, Wied. Ann., Ix, p. 418, 1897. 

 JH. Rubeus, Wied Ann., liv, p. 476, 1895. 

 § P. Paschen, Wied. Ann., liii, p. 301, 1894. 



Am. Joue. Sci.— Fourth Series, Vol. V, No. 25.— Jan., 1898. 

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