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XLIII. On the Dispersion of Light in Gases. 

 By Dr. Ketteler*. 



THE livelier the interest becomes which is now taken in the 

 search after the laws whereby the propagation of light is 

 connected with the density and chemical composition of the me- 

 dium in which it moves, the more firmly shall we be convinced 

 that it is only through a comprehensive study of the phenomena 

 of refraction and dispersion in gaseous bodies that the discovery 

 of comparatively simple laws is possible, and that the true path 

 by which to approach the incomparably more complex case of 

 solid and liquid bodies lies through these. I have therefore un- 

 dertaken to submit the refracting and dispersive powers of gases 

 to a more accurate investigation. The essential parts of the 

 physical portion of this investigation are already completed, and 

 I therefore communicate in the following pages the results that 

 have been arrived at. 



Thanks to the method I adopted, I have succeeded in deter- 

 mining with precision the indices of refraction of the gases for 

 the separate Fraunhofer's lines, and in ascertaining the law which 

 connects these indices with the density. I have indeed aban- 

 doned the method of prismatic analysis, which has hitherto been 

 adopted almost exclusively, and based my apparatus on the prin- 

 ciple of interference. If it be borne in mind that the excess of 

 the index of refraction above unity, which becomes in the interfe- 

 rential method the object of direct observation, is, according to 

 the older experiments of Arago and Biot, proportional to the 

 density — if it be considered, further, that the advantage just men- 

 tioned is of still greater importance in the study of dispersion, 

 since exact coincidences of differently coloured fringes are very 

 easily established, and are moreover comparatively little affected 

 by changes of form in the closing plates, whereas the accurate 

 adjustment of an instrument upon the lines of a spectrum whose 

 entire length is measured by seconds must be nearly impossible, 

 it will be admitted that the method that has been adopted is the 

 only appropriate one. Sources of homogeneous light of various 

 colours were of course necessary; and the fringes had to be fol- 

 lowed, when needful, by thousands, without the interposition of 

 compensators. It will be seen that such a mode of operating 

 could not have been practically available but for the discovery of 

 Professors Bunsen and Kirchhoff. 



The following experiments were made for the most part in the 

 new Institute of Natural Science at Heidelberg, where Professor 

 Kirchhoff most liberally placed at my disposal the requisite space 

 and apparatus, without which this difficult investigation would 

 not have been possible. 



* Translated from Poggendorff's Annalen, vol. cxxiv. p. 390, with cor- 

 rections commupicated by the Author. 



