Scientific American, 25.11.1869
For the Scientific American.
In a report of the proceedings of the Royal Astronomical Society, published in May last, there is a record of several interesting observations, concerning the spectrum lines of Aurora, which it is interesting to compare with several observations made on this side of the Atlantic Ocean. These observations promise to be useful in aiding us to determine the nature of the Northern Light.
In the report alluded to, Mr. Plumber tells us, that in the spectrum of Aurora, he saw one bright line in the green, near E.
Mr. Angström saw it as one bright line in the yellow, near D, and several faint bands, near F.
Mr. Struve observed one bright line, near D, and traces of two others in the green.
Professor Winlock has seen six lines,the brightest of which was near E.
The writer has frequently seen one bright line in the yellow, near D (coincident with one of a group of lines which. appear in the solar spectrum, when the sun is near the horizon), and one faint line in the green. On one occasion there was visible one additional line in the red.
It has always proved a difficult task to determine, with certainty, the position of the spectrum lines of Aurora, and as the value of observations with the spectroscope rests principally upon our ability to do so, I am glad to find that the locations of eight lines have been announced.
The wave length of M. Angström's bright line is 556.7.
The lines seen by Mr. Winlock, he determines, micrometrically to be as follows: the bright line 1474, the other five lines, 1280, 1400, 1510, 1680, 2640, Kirchoff's scale.
The bright line seen by myself I found to be very nearly 557.
Now we learn from these observations: First, that the light, of Aurora gives a spectrum consisting of bright lines; secondly, that the same number of lines are not always seen; thirdly, that the lines are fixed in their positions; fourthly, that the same line is not always the brightest; that one line in the spectrum of Aurora is coincident with a dark line, which appears in the solar spectrum, when the sun is near the horizon.
I was much pleased to find in No. 15, current volume, SCIENTIFIC AMERICAN, an interesting letter from Professor Vander Weyde, criticising the conclusions reached by M. Angström, and, also, those resulting from my own observations. To the objections which he urges against my hypothesis will reply briefly, and, I trust, in the same kind spirit which he has shown in his criticism.
First, he objects because the spectrum seen by me is different from the spectrum of oxygen.
I reply, that this is a weighty objection to the opinion I have expressed, that Polar light is principally incandescent oxygen. But I have been led to this conclusion from the coincidence of the bright line in Aurora, with a line in Solar light, which, I think it probable, is produced by oxygen, because of the density of that gas. The difference between the spectrum of oxygen and that of Aurora, does net seem necessarily to prove my opinion incorrect, for it is a well-known fact, that the spectra of elements vary according to the circumstances under which they are produced. For illustration, potassium usually gives a spectrum of only three of the seventeen lines of which it is known to consist. Again, the position of the hydrogen line, F, in the spectrum of Sirius is changed by the movement of the star, as it recedes from the earth. Again, carbon gives six differing spectra, according to the circumstances under which they are produced, and in these the same line is not always the brightest.
Secondly, Professor Vander Weyde objects, because of the presence of a line, in the spectrum, that has not been identified. I confess that I am at a loss to comprehend this argument,as I have only expressed the opinion that Auroral light is, principally, not exclusively, incandescent oxygen.
Lastly, he objects to my explanation of the change of the bright line to a black one. I reply. that I accept the common theory, explaining the change of solar lines from bright to dark ones; I never, for a moment, doubted it; but the line under consideration is not an ordinary solar line, but one that is seen only when the sun is near the horizon, and, therefore, seems to require a different explanation„ and as it is not seen at midday, I conclude that it is darkened by absorption in its passage (morning and evening) through the earth's atmosphere.
I am happy to find so many distinguished scientific gentlemen interested in the subject of the nature of Aurora Borealis, and I entertain a hope that the observations made before the present season of Auroral displays shall have passed away, will enable us to explain more fully the nature of its phenomena.
Toronto, Ont., Nov. 15, 1869.
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