46 M. A. Wiillner on the Absorption of Light. 



annate in red, yellow, and green. Blue and violet appear en- 

 feebled, it is true, especially if the iodine vapours were very 

 dense ; but bands could not be detected in this part of the 

 spectrum. Corresponding to this, only red, yellow, and green 

 showed themselves in the spectrum of iodine, and scarcely a trace 

 of blue and violet. A similar agreement was observed between 

 the spectra of bromine and hyponitric acid vapours, and the 

 absorption spectra of these vapours. 



With iodine vapour it was possible to recognize bright and 

 dark bands in the spectrum of the flame, but not to furnish the 

 proof that the bright bands of the flame corresponded to the dark 

 lines of the absorption spectrum and inversely, since in both 

 spectra the position of the bands could not be ascertained by my 

 apparatus. 



In the other flames no bands could be observed, since their 

 light was too feeble to bear the necessary contraction of the slit. 



The observation of the iodine spectrum acquired for me an 

 additional interest after having had, in Bonn a few weeks ago, 

 the opportunity of witnessing the beautiful observations of M. 

 Pliicker on the spectra of the gases, which that gentleman had 

 the goodness to show me. When I saw the nitrogen spectrum, 

 designated " the first '■ by M. Pliicker, T at once recognized that 

 the iodine spectrum which I had seen was not the second iodine 

 spectrum previously observed by M. Pliicker, but a first one, 

 which M. Pliicker since his discovery of the double spectra had 

 predicted, and in the observation of which he expected the same 

 proof for the new theory*. 



* In reference to the observation of M. Pliicker on the first and second 

 spectra of incandescent gases, I communicate, with M. Pliicker's permis- 

 sion, the contents of a letter which he sent me in answer to an epis- 

 tolary communication of the following observations. M. Pliicker writes, 

 — "■ In the well-known historical notice of Dr. Miller in London, ' On 

 Spectrum Analysis,' the objection is made to my theory of the absorption 

 of light that the iodine spectrum, which I had determined in 1859, stands 

 in no relation to that obtained if sunlight (or, better, light which for itself 

 gives a continuous spectrum) is passed through iodine vapour. The spec- 

 trum of iodine consists of bright lines on a dark background ; the absorp- 

 tion spectrum, of about fifty compartments which are peculiarly shaded by 

 dark lines. Spectra of the first kind I have subsequently designated as 

 ' second spectra '; spectra like the one in question, ' first spectra,' to which 

 the nitrogen spectrum, described in 1858, also belongs. 



" From the investigations made in the physical laboratory of this place in 

 August last year, by Professor Hittorf and myself in common, it was found 

 that certain bodies, which at present it would be rash to say were not simple 

 (nitrogen, sulphur, &c), give, according to the temperature, two absolutely 

 different spectra — a first one that corresponds to the lower, and a second 

 one that corresponds to the higher temperature. The transition from the 

 one to the other is discontinuous. In consequence of this novel and 

 unforeseen fact, Miller's objection appeared to me under a new point of 





