320 Dr. A. Schuster on Spectra of Lightning. 



the centres of the bands under these circumstances, I obtain 

 \=5579 and \=5180, which agree within the limits of pos- 

 sible errors with the above values. 



The ordinary spectrum of air, however, contains a band at 

 5178; so that, as far as mere position is concerned, one might 

 well be taken for the other. I was, however, under the im- 

 pression that I had sometimes seen this band without the chief 

 nitrogen double line 5002-5; and as the yellow band of car- 

 bonic oxide was also apparently present, I stated with consi- 

 derable confidence when I first wrote out this paper that I had 

 observed the spectrum of carbonic oxide. It was only when 

 I came to work out the position of the band 8 that I began to 

 have serious doubts as to the accuracy of this conclusion. The 

 position of the band 8, as I have said, is very doubtful ; I even 

 thought it was possible that I had taken a very bad measure- 

 ment of either or y, and felt at first inclined to reject it alto- 

 gether. On working out its wave-length, however, I found 

 that it was coincident with one of two strong bands, which are 

 found at the negative pole of vacuum-tubes filled with oxygen. 

 Now the second of the two bands is nearly coincident with 

 the yellow band of carbonic oxide ; so that, of the two bands 

 which I at first thought belonged to that gas, one might be 

 due to nitrogen, the other to oxygen, as seen at the negative 

 pole. 



The explanation of the band /3 is obvious. It is the brightest 

 of the two green lines in the low-temperature spectrum of 

 oxygen. Its wave-length, when seen under a pressure of about 

 a millimetre, is 5329 ; but under higher pressures it widens 

 more on the less-refrangible side than towards the blue, and 

 may well appear as a band with its centre at 5334 or even 

 5341, as given by Vogel. 



I have not been able to obtain this band from atmospheric 

 air in vacuum-tubes, although I have tried the experiment 

 under various pressures. If the so-called continuous discharge 

 is allowed to pass, the band spectrum of nitrogen alone appears; 

 if the disruptive discharge passes, the high-temperature spec- 

 trum of oxygen is superadded to the line spectrum of nitrogen. 

 As regards the two bands a and 7, it does not seem to me to 

 be possible at present to decide between the two interpreta- 

 tions which I have given. On the one hand, it seems impro- 

 bable that the slight traces of carbonic acid known to exist in 

 the atmosphere should reveal their presence in the spectrum ; 

 but, on the other hand, it is to be remarked that oxygen 

 vacuum-tubes, which show the band /3, always reveal the 

 slightest trace of carbonic oxide. It is exceedingly difficult, 

 though quite possible, to obtain the band/3 without the bands 



