CHROMOSPHERE, the gaseous envelope which exists round the body of the sun, through wtuch the light of the photosphere, an inner envelope of incandescent matter, passes. Dur ing total eclipses it is observed that a red-col ored envelope surrounds the sun and shoots up to great distances from the surface. It seems to have been first recognized by Secchi; and the projecting portions of it are commonly de scribed as "red-colored protuberances' and °red flames.' To this red envelope the name °chromosphere° was given by Lockyer. The light from it is much fainter than that from the photosphere; and till 1868, when M. Jans sen and Mr. Lockyer almost simultaneously pointed out a of viewing it, it was never seen except during eclipses. See SUN.
The spectrum of the chromosphere was first observed in 1868 during the Indian total eclipse, and it was found to consist of a num ber of bright lines, and conspicuous among them those of hydrogen. The light of the chromosphere was thus proved to be due to masses of incandescent vapor or gas, hydro gen forming a large part of the whole.
Since the invention of the Janssen-Lockyer method of observing, as it is called, very re markable advances have been made in our knowledge of solar physics, discoveries quite unthought-of having • followed. The observa tions are made by means of a combined tele scope and spectroscope. Aroscope is sub stituted for the eyepiece of the telescope, the slit of the spectroscope being placed at the principal focus of the object-glass of the tele scope. The slit is capable of being moved in such a way that any particular band of the image formed in the object-glass of the tele scope may be examined by the spectroscope.
The spectroscope employed for the purpose of examining the chromosphere must have the greatest possible dispersive power, and requires for this purpose a very long train of prisms.
As was mentioned above, the spectrum of the chromosphere consists of a series of bright lines. The breadth of a bright line of the spec trum is not sensibly increased by increasing the dispersive power of the spectroscope, but the contrary is the case with a continuous spec trum, which is extended by dispersion. Thus
the latter becomes weakened, while the former maintain their brightness and become more easily visible in comparison with it. The slit of the spectroscope being arranged so as to take in a band, either tangential or radial, close to the edge of the image of the sun formed by the object-glass of the telescope, it is found that the bright lines of the chromo sphere are perfectly visible, in spite of the light of the continuous spectrum proceeding from the inner portion of the disc. Further it has been found possible, by using a spectro scope of the very highest dispersive power, and by opening the slit sufficiently wide, to see the whole of one of the protuberances at once, and by this means to watch its motions and its changes. To understand this the reader must consult the article SPECTROSCOPE. He must rec ollect that a continuous spectrum consists of an enormous number of images of the slit, placed side by side, and in ordinary cases slightly over lapping each other. If we could employ an in finitely narrow slit we should have an infinite number of infinitely narrow images, and no overlapping whatever. But suppose a light to consist of only two or three colors, say light from a source capable only of giving the two bright lines C and F that is, one in the red and another in the blue part of the spectrum. It is easy, even with a slit of sensible width, to keep the two from overlapping, and we shall see without any confusion the two bright lines or bands at different parts of the spectrum, darkness intervening. Now imagine a bright prominence starting up from the sun's surface, and let the spectroscope be directed on its im age in the telescope; there will be only portions of the slit illuminated by it, portions corre sponding to the shape of the flame, and if the flame contain only light of the bright line C and F, there will be seen two images of the prominence at the points of the spectrum be longing to these colors.