( 707 ) 



dispersion curve of rock-sail is nol yet known with sufficient exactness. 



Mr. W. .1. II. Moll 1 ) has lately compared with each other the 

 dispersion curves that have been calculated according to KettklkrV 

 formula with two sets of constants, one given l>y Rubens *), the 

 other by Langlky 8 ). Langley's results held for a temperature 

 of 2(F; the numbers given by Rubens were corrected by Mr. Moll 

 so as to apply to the same temperature. While coinciding in the 

 visible spectrum, the two dispersion curves appeared to diverge very 

 sensibly in the entire infra-red region, the wave-lengths correspond- 

 ing to given indices of refraction being smaller with Rubens' than with 

 Langley's constants. At P. = 1,5»< e.g. the difference amounts to 

 0,028 ft; it increases unto 0,062 fi (at ?. = 3 ft) and then decreases to 

 0.032 ft (at ;. = 8,5 ft). If, on the other hand, the indices of refrac- 

 tion, which according to Langley's and according to Rubens' formula 

 belong to rays of given wave-lengths, be compared with each other, 

 the difference appears to be rather constant between X = 4 ft and 

 X = 8,3 ft, namely 1,5 units of the 4 th decimal of the index, and to 

 increase from to 1.5 similar units in the region between 0.6 ft 

 and 4 ft. 



The apparatus, nowadays available for the investigation of the 

 infra-red, admit of determining the position of sharp maxima or 

 minima of radiation with an accuracy, going a good deal farther 

 than 1,5 units of the 4 ,h decimal of the index. 



When between 'J 887 and 1891 I investigated several infra-red 

 emission and absorption spectra, our knowledge of the dispersion of 

 rock-salt was restrained to the outcome of Langley's first determi- 

 nations 4 ), which extended only as far as 5,3 ft. As a great part of 

 my work bore upon longer waves, I published my results in the 

 form given by direct observation, viz, as galvanometer deflections 

 and corresponding angles of minimum deviation, reduced to the 

 temperature 10°. The refracting angle of the prism being also recorded, 

 the indices of refraction of rock-salt for waves, corresponding to the 

 observed maxima, were thus implicitly given. 



In order to obtain a rough estimate of the wave-lengths, I had 

 extended Langley's dispersion curve in a straight line, though under 

 strict reservation. The wave-lengths as read on this lengthened 



1 ) W. J. H. Moll, Onderzoek van ultra-roode spectra. Dissertation, Utrecht, 1907. 



2) H. Rubens, Wied. Ann. 60, 724; 61, 224; 1807. Cf. also Kayser, Handbuch 

 der Spectroscopie I, 371, 1900. 



3 ) S. P. Langley, Ann. Astroph. Obs of the Smiths. Inst. I. 1900. 

 ±) S. P. Langley, Phil. Mag., Aug. 1886. 



