SPECTROHELIOGRAPHIC INVESTIGATION It has been said that the absorption lines in the solar spectrum contain light emitted by the glowing atmospheric gases; it is the photospheric light that is absorbed. The spectroheliograph is an instrument which gathers the light in a particular spectrum line and builds up a picture of the solar atmosphere in the light of that line, showing thereby how the substance producing the line is distributed in the solar atmosphere. The construction of the instrument is explained in the article SPECTROHELIOGRAPH. Some of the results which it produces are shown in Plate I., figs. 4 and 5.
It is clear that the evidence of the spectroscope and spectro heliograph points the way to a fairly complete knowledge of solar meteorology, from which it appears that the structure of the solar atmosphere is anything but uniform. Atoms congregate in clouds, or flocculi, particularly in the regions of sunspots, where they form the faculae observed through the telescope. Different
substances are differently distributed, and the distribution of each substance is in a state of continual flux. The restless tur moil of the solar atmosphere stands in striking contrast to the comparative quietude and regularity of our own air. Yet even in the sun the motions are not wholly chaotic. On the large scale there is a consistency which points to a fundamental equilibrium. There is a fairly well defined limit to the ascent of each substance above the photosphere; we never find sodium so high as hydrogen, for example. Spectroheliograms are markedly characteristic in appearance for each substance; the coarse mottled structure of the calcium clouds is immediately distinguishable from the finer grained layers of hydrogen, inconstant as each of them may be in detail. Once more there is a foundation of law and order to the superficial irresponsibility.