The phenomena exhibited by the corona in different eclipses are very complex, and, so far, they have not been sufficiently studied to enable us to understand the true nature of the solar atmosphere. From the spectral analysis of the corona, and the phenomena of polarization, it has been learned, at least, that while the matter composing the upper part of the solar atmosphere is chiefly composed of an unknown substance, producing the green line 1474, its lower part is mainly composed of hydrogen gas at different temperatures, a part of which is self-luminous, while the other part only reflects the solar light. But the proportion of the gaseous particles emitting light, to those simply reflecting it, is subject to considerable variations in different eclipses. At present it would seem that in years of great solar disturbances, the particles emitting light are found in greater quantity in the corona than those reflecting it; but further observations will be required to confirm these views. It is very difficult to understand how the corona, which in certain eclipses extends only one diameter of the Sun, should, in other cases, as in 1878, extend to the enormous distance of twelve times the same diameter. Changes of such magnitude in the solar atmosphere, if due to the operation of forces with which we are acquainted, cannot yet be accounted for by what is known of such forces. Their causes are still as mysterious as those concerned in the production of the monstrous tails displayed by some comets on their approach to the Sun. Plate 3, representing the total eclipse of the Sun of July 29th, 1878, was drawn from my observations made at Creston, Wyoming Territory, for the Naval Observatory. The eclipse is represented as seen in a refracting telescope, having an aperture of 6⅓ inches, and as it appeared a few seconds before totality was over, and when the chromosphere was visible on the western limb of the Sun. The two long wings seen on the east and west side of the Sun, appeared considerably larger in the sky than they are represented in the picture. THE AURORA BOREALIS PLATE IV The name of Polar Auroras is given to certain very remarkable luminous meteoric phenomena which appear at intervals above the northern or the southern horizons of both hemispheres of the Earth. When the phenomenon is produced in our northern sky, it is called "Aurora Borealis," or "Northern Lights;" and when it appears in the southern sky, it is called "Aurora Australis," or southern aurora. Marked differences appear in the various auroras observed from our northern latitudes. While some simply consist in a pale, faint luminosity, hardly distinguishable from twilight, others present the most gorgeous and remarkable effects of brightness and colors.