The bearing of these observations as connected with the variable character of the red line in the Aurora-spectrum seems to me in the highest degree noteworthy.
Iron-Spectrum.
Iron-spectrum.
A comparison of this spectrum suggested itself, partly from the suspected relations between the Aurora and solar corona, and partly from a consideration of the views expressed by M. Gronemann and others in favour of the Aurora having its origin in the fall of an incandescent meteoric powder.
How obtained. Spectrum described. Mons. Lecoq de Boisbaudran’s spectra also given.
The spectrum was obtained from a spark taken over a solution of perchloride of iron in a small glass cup, and was remarkable for its brightness in and about the green region. The lines varied considerably in intensity, and with a fine slit the principal ones were sharp, distinct, and clear. A group of three lines (α) stood out boldly in the green as the most marked, and next to these a group of three others more towards the violet end of the spectrum (see Plate XVI. spectrum 4). By the side of my phosphoretted-hydrogen and iron spectra I have placed the principal lines of Mons. Lecoq de Boisbaudran’s same spectra (reduced to my scale), and with figures of wave-lengths for comparison with the Aurora-spectrum (see Plate XVI. spectra 1 and 2).
Comparison of iron- and Aurora-spectrum.
A difficulty in comparing the iron-spectrum with that of the Aurora arises from the large number of fine lines found in the former spectrum. In a photograph (taken with the same prism as before described) of a small piece of meteoric iron fused in an electric arc by the aid of 40 Grove cells, about 154 lines are easily counted in the blue and violet parts of the spectrum. Double this number at least would be seen with a spectroscope of moderate dispersion in the region comprising the entire set of auroral lines.