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). [Sidenote: 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. _Spectrum of Mercury._ [Sidenote: Mercury-spectrum. How obtained.] This spectrum is given as useful for comparison with the bright and principal Aurora-line. It is easy to obtain with a small coil, the metal being used as one electrode. The yellow lines are distinct and steady; but the green, which is very bright, is apt to flicker as the spark moves on the surface of the metal (see Plate XVI. spectrum 5). _The following Table was compiled for the purpose of comparing the foregoing results with the Aurora-spectrum._ [Sidenote: Table of coincidences.] TABLE showing comparative position of Aurora-lines with the principal lines in the examined spectra. C. means coincident within the limits of my instrument and scale, N. near, and VN. very near.