CHROMOSPHERE AND PROMINENCES.
The Chromosphere, so called on account of its colour, is a gaseous envelope three or four thousand miles deep, surrounding the hotter part of the sun. Like the corona, it can be advantageously photo graphed at the time of an eclipse. It gives out a rosy light, which is supposed to be due to hydrogen, and has a bright line spectrum, consisting principally of hydrogen lines. One well-defined line arrests the attention ; it has been called D,, and as it could not be matched by any line in the spectrum of a terrestrial sub stance, the unknown element giving out that particular coloured light was called " helium," from Greek " helios," the sun. But, lately, a gas has been obtained from a mineral called " cleveite," which con tained this line in its spectrum, by Sir W. Ramsay. Bruggerite also contains this element white paper be looked at, the colours will not come in the usual order, but as follows : Yellowish green, yellow, red, violet, indigo, blue, and green. This is
because the ring will most probably be darker than the paper, in which case the spectrum is produced by the bright edges of the paper adjacent to the ring. The foregoing method of observing the spec trum, the principle of which was first enunciated by Fraunhofer, is extremely simple. The whole working apparatus consists of one or two prisms placed in front of the object glass of a telescope. This instrument, which is much employed in eclipses, is termed a coronagraph. (The corona is photographed along with the chromosphere.) The chromosphere acts as a slit, and needs no collimator.
H13, and Hy are represented by three rings, D, by another, and so on. For comparison, line spectra may be made from these negatives by using a small strip of the ring spectrum and a cylindrical lens. Very often the object glass is corrected for photographic rays, when the focus must be found by photography.