M. A. Wiillner on the Absorption of Light, 49 



evident. But even then the dark bands could not be fixed and 

 their position in the scale determined, since the phenomenon 

 was altogether too indistinct, and since it was almost impossible 

 to keep the flame longer in this position, on account of the 

 difficulty, with a spirit-lamp, of regulating the temperature of 

 the iodine-tube so that not too much and not too little iodine 

 may pass into the flame*. 



It was also difficult to determine the more distinct bands in 

 the red and yellow, because a good spectrum could only be 

 obtained for a short time ; yet in a number of experiments their 

 position could be determined from about C to something beyond 

 the part of the spectrum corresponding to D. The following 

 small Table contains the dark bands of the flame spectrum whose 

 position could be determined with certainty. 



The dark bands in the flame spectrum lay upon and between 

 the divisions of the scale 



70—71 



84 



72—73 



86 



74—75 



88 



76—77 



91—92 



78 



93—94 



82 



95—96 



The links between the numbers 78 and 82, and between 88 

 and 91, do not mean that there are no dark bands there; they 

 depended on the fact that the last observations, which would 

 have given me the bands between 80 and 90, had to be inter- 

 rupted owing to an unfortunate accident. 



If the dark bands in the absorption spectrum of daylight 

 which has passed through iodine vapour be compared with those 

 in the flame spectrum, it is seen, in the most surprising manner, 

 that the dark bands in the flame spectrum are where the absorp- 

 tion spectrum has bright lines, and inversely ; for the dark 

 bands lay upon and between the divisions — 



* As M. Plucker tells me, in the letter mentioned above, he has in just 

 such an iodine flame recognized the iodine spectrum corresponding to the 

 absorption spectrum. He writes : — 



" From the beginning of my investigations, when with the aid of an in- 

 duction current 1 obtained the spectrum of an enclosed rarefied gas, I 

 designated this phenomenon as one solely depending on the heating by 

 the current, and proposed to myself to make the gas incandescent in an- 

 other way. The older experiments, especially upon iodine, were imperfect ; 

 new experiments were already commenced at the time of your visit ; the 

 arrangements which M. Ste. -Claire Deville uses for producing high tempe- 

 ratures were in my hands. Since your departure I have, as a preliminary 

 experiment, prepared the first spectrum of iodine in the manner in which 

 you proceeded, and which we discussed." 



Phil. Mag. S. 4. Vol. 27. No. 179. Jan. 1864. E 



