SPECTROSCOPIC INVESTIGATION General Considerations.—The spectroscope is an instrument by means of which a beam of light is analysed into its constituent colours, or wave-lengths. The analysed radiation of a substance is called its spectrum, and the instrument is constructed to show each colour radiated as a thin vertical line ; hence the term spectrum line is often used to denote a particular colour. When, in the laboratory, a substance is vaporised and made luminous, the light it emits, when analysed by the spectroscope, appears as a collection of isolated lines and is characteristic of the substance. On the other hand, a glowing solid, liquid, or gas under great pressure or of great depth, such as is found in the stars, radiates light of all wave-lengths (within rather ill-defined limits), appear ing in the spectroscope as a continuous band of colour ranging from red to violet in the order of the colours of the rainbow. This is known as a continuous spectrum. Again, when a beam of light which, if analysed, would form a continuous spectrum, is allowed to pass through a less brightly glowing vapour which, acting alone, would give a line spectrum, and is then received by the spectro scope, the spectrum formed consists of a continuous background on which dark lines appear exactly in the positions of the bright lines which the interposed vapour would give by itself. This is called an absorption spectrum. The glowing vapour absorbs, from the light passing through it, precisely those colours which it can itself emit. Absorption lines are not absolutely dark. They contain the light emitted by the glowing vapour.
The Solar Spectrum.—The spectrum of the sun is an absorption spectrum. This gives us immediately a piece of knowledge which the telescope was powerless to reveal; namely, that the sun has an atmosphere of glowing vapours surrounding the brilliant photo sphere. Further, it enables us to ascertain the elements which make up this atmosphere by simply comparipg the positions or wave-lengths, of the absorption lines with those of the emission lines produced by known substances in the laboratory. In this manner the presence in the solar atmosphere of 66 elements known on the earth (including a few doubtful identifications) has been established. The remaining elements are mainly the
heaviest ones, which might be expected to sink to the interior of the sun.
The existence of the solar atmosphere is revealed in another way during times of total eclipse. When the moon has just covered the last remnant of photosphere, the crescent of the atmosphere which still remains exposed for a few seconds gives a bright line spectrum, for the glowing vapours there have no bright photosphere behind them. On account of its evanescence, this spectrum is known as the flash, and it is, of course, essentially identical with the Fraunhofer, or absorption, spectrum in the positions of its lines. Careful observation at such times enables the heights reached by the various substances to be determined. It is found that the majority are confined to the lowest atmos pheric layer, not more than 500 miles high, known as the reversing layer. Hydrogen, helium, and calcium charged with positive electricity (ionised calcium), however, reach much greater heights, extending up to 8,000 or 9,000 miles, and form a relatively thick upper stratum of the atmosphere, which is known as the chromosphere on account of the red colour imparted to it by the glowing hydrogen. Prominences are eruptions of hydrogen.
The portion of the solar atmosphere just outside the sun's limb might be expected to give a bright line spectrum in full daylight. It does not do so, however, because of the diffused photospheric light in the earth's atmosphere which masks the bright radiations. Pierre Jules Cesar Janssen and Joseph Norman Lockyer, in 1868, independently succeeded in partially overcoming this difficulty. If the dispersion of the spectroscope (i.e., its power of separating the colours) is increased, the intensity of the continuous spectrum at any one point is diminished, while the lines in a bright line spectrum are merely drawn farther apart without loss of bright ness. Janssen and Lockyer found it possible so to weaken the diffused daylight that the bright lines of the chromosphere and prominences became visible.