The luminous surface of the Sun, or that part visible at all times, and which forms its disk, is called the _Photosphere_, from the property it is supposed to possess of generating light. The photosphere does not extend to a great depth below the luminous surface, but forms a comparatively thin shell, 3,000 or 4,000 miles thick, which is distinct from the interior parts, above which it seems to be kept in suspense by internal forces. From the observations of some astronomers it would appear that the diameter of the photosphere is subject to slight variations, and, therefore, that the solar diameter is not a constant quantity. From the nature of this envelope, such a result does not seem at all impossible, but rather probable. Immediately above the photosphere lies a comparatively thin stratum, less than a thousand miles in thickness, called the _Reversing Layer_. This stratum is composed of metallic vapors, which, by absorbing the light of particular refrangibilities emanating from the photosphere below, produces the dark Fraunhofer lines of the solar spectrum. Above the reversing layer, and resting immediately upon it, is a shallow, semi-transparent gaseous layer, which has been called the _Chromosphere_, from the fine tints which it exhibits during total eclipses of the Sun, in contrast with the colorless white light radiated by the photosphere below. Although visible to a certain extent on the disk, the chromosphere is totally invisible on the limb, except with the spectroscope, and during eclipses, on account of the nature of its light, which is mainly monochromatic, and too feeble, compared with that emitted by the photosphere, to be seen. The chromospheric layer, which has a thickness of from 3,000 to 4,000 miles, is uneven, and is usually upheaved in certain regions, its matter being transported to considerable elevations above its general surface, apparently by some internal forces. The portions of the chromosphere thus lifted up, form curious and complicated figures, which are known under the names of _Solar Protuberances, or Solar Flames_. Above the chromosphere, and rising to an immense but unknown height, is the solar atmosphere proper, which is only visible during total eclipses of the Sun, and which then surrounds the dark body of the Moon with the beautiful rays and glorious nimbus, called the _Corona_. These four envelopes: the photosphere, the reversing layer, the chromosphere, and the corona, constitute the outer portions of our luminary. Below the photosphere little can be seen, although it is known, as will appear below, that at certain depths cloud-like forms exist, and freely float in an interior atmosphere of invisible gases. Beyond this all is mystery, and belongs to the domain of hypothesis. STRUCTURE OF THE PHOTOSPHERE AND CHROMOSPHERE