estimation than by measurement, the coincidence between the bright lines of the corona and those of the Aurora would be found very remarkable. The brightest of the lines, 1474, was the reversal of a strongly marked Fraunhofer line, ascribed by Kirchhoff and Ångström to the vapour of iron. Dr. Schellen then details Professor Pickering’s observations with the polariscope, showing that the corona must be self-luminous, and that from the bright lines seen in its spectrum it is probably of a gaseous nature, and forms a widely diffused atmosphere round the sun; and then adds, “It has been supposed, from the coincidence of the three bright lines of the corona with those of the Aurora Borealis, that the corona is a permanent polar light existing in the sun analogous to that of our earth.” Dr. Schellen here adds:—“Lockyer, however, justly urges against this theory the fact that although the brightest of these three lines, which is due to the vapour of iron, is very often present among the great number of bright lines occasionally seen in the spectrum of the prominences, it is by no means constantly visible, which ought to be the case if the corona were a permanent polar light in the sun.” (Professor Young’s answer to this, on the ground of line 1474 being always visible, has been already given.) “A yet bolder theory is the ascription of such a polar light in the sun to the influence of electricity, which has been proved, it is well known, by the relation of the magnetic needle, and the disturbance of the electric current in the telegraph wires, to play an important part in the phenomenon of the Aurora Borealis;” and Dr. Schellen then concludes with an opinion that the nature of the corona was still a problem[12]. [Sidenote: Various places of wave-length assigned to these lines.] On reference to the ‘American Journal of Science,’ vol. xlviii. pp. 123 and 404, it seems that the auroral observations before referred to were made on 15th April, 1869, by C. S. Pierce, with “an ordinary chemical spectroscope, with the collimator pointed directly to the heavens,” and were reported by Winlock. The lines were 1280, 1400, and 1550 of Huggins’s scale, and were reduced to Kirchhoff’s scale by Young. These lines have had all sorts of places of wave-length assigned to them by different writers. Proctor gives 5570, 5400, 5200; Pickering and Alvan Clarke, 5320 (assumed to be 5316, coronal line); Barker, 5170, 5200, 5020; Backhouse, 5320, 4640, and 4310. In my ‘Aurora Spectrum,’ Plate XII., I have assigned two, with a?, to 5320 (Alvan Clarke) and 5020 (Barker). The third might perhaps be placed at 4640 (Backhouse and Winlock). [Sidenote: Doubts raised as to closeness of the observations for the purpose of comparison.]