. ( 708 ) 



curve, to which I myself assigned lillle weight 1 ), have found their 

 way to some text-books 2 ), where they unfortunately appear as the 

 results of my investigation, with the incidental remark that they are 

 incorrect, as founded on a false extrapolation. It is clear, however, 

 that this incorrectness has nothing to do with the accuracy with 

 which the position of the bands in the prismatic spectrum has been 

 determined. Now I have reason to believe, that the spectrometric and 

 heat-measuring apparatus used in that research were not less valid 

 than those employed by many later observers of infra-red spectra 



(DONATH, PUCCIANTI, IkLE, CoBLENTZ, NlCHOLS, RüBENS and ASCHKINASS 



and others), so that the results still retain their value as a first 

 contribution to our knowledge of the examined spectra. 



I therefore thought it suitable to republish the principal results 

 obtained at that time 3 ), but now to mention the indices of refraction 

 for the maxima of emission and absorption, as following directly 

 from my observations, and to add the wave-lengths, as derived from 

 the more recent dispersion curves of Rubens and of Langley. 



The positions in the infra-red were determined in my work with 

 respect to the place of the /Mines of a Bünsen flame coloured with 

 chloride of sodium. But the latter were too faint to be observed with 

 the bolometer; and the transition from the visual observation of the 

 D-lines to the bolometric observation of infra-red radiations caused 

 an uncertainty in the determination of the relative positions, which 

 was still increased through the necessity of displacing the bolometer 

 along the optical axis of the rock-salt lens according to its different 

 focus for visible and invisible rays. It was chiefly in the part of 



1 ) Cf. „Bolometrisch onderzoek van absorptie spectra", Verhandelingen der Kon. 

 Akad. v. W. te Amsterdam, Vol I, N°. 1, p. 8 (1892), or the German translation 

 in: Verhandl. des Vereins zur Beförderung des Gewerbfleisses, 1893, p. 235, where 

 1 have clearly stated that 1 considered the extrapolation of Langley's dispersion 

 curve as quite uncertain, and that in the tables the direct data of observation 

 (angles of minimum deviation) were given, because I did not like to have my 

 results inseparably connected with a possible incorrectness of the dispersion curve. 

 The passage in question seems not to have been noticed by W. W. Goblentz, for 

 in his excellent work 'Investigations of Infra-red Spectra", published by the 

 Carnegie Inst, of Washington, 1905, he says on p. 135, after alluding to Langley's 

 extrapolation of the dispersion curve in a straight line: "Julius, with apparently 

 less hesitation, has applied this extrapolation to his work". 



2 ) Winkelmann, Handbuch der Physik ; Kayser, Handbuch der Spectroscopie ; 

 Chwolson, Lehrbuch der Physik. 



3 ) Recherches bolométriques dans le spectre infra-rouge. Arch, neérl. 22, p. 

 310-383 (1888). 



Die Licht- und Warmestrahlung verbrannter Gase, Berlin, Simion. 1890. 

 Bolometrisch onderzoek van absorptiespectra, 1. c. 



