When the dark body of the moon has entirely hidden the sun, we have revealed to us, there and then only, that strange and beautiful surrounding of the sun which we call the corona. The earlier observations of the corona seem to reveal it as a body of the most weird and intricate form, a form which seemed to change quite lawlessly from one eclipse to another. But latterly it has been abundantly clear that the forms which it assumes may be grouped under a few well-defined types. In 1878 the corona was of a particularly simple and striking character. Two great wings shot out east and west in the direction of the sun's equator; round either pole was a cluster of beautiful radiating 'plumes.' It was then recollected that the corona of 1867 had been of precisely the same character, both years being years when sun-spots were at their fewest. The coronæ, on the other hand, seen at times when sun-spots are more abundant, were of an altogether different character, the streamers being irregularly distributed all round the sun. Other types also have been recognized, and it is perfectly apparent that the corona changes its shape in close accordance with the eleven-year period. The eclipses of 1889 and 1900, for example, showed coronæ that bore the very closest resemblance to those of 1878 and 1866, the interval of eleven years bringing a return to the same form. The further problem, therefore, now confronts us: Does the corona produce the sun-spots, or do the sun-spots produce the corona, or are both the result of some mysterious magnetic action of the sun, an action powerful enough on occasion to thrill through and through this world of ours, ninety-three millions of miles away? CHAPTER XI THE SPECTROSCOPIC DEPARTMENT Another department was set on foot by Airy at the same time as the Heliographic Department, and in connection with it; and it is the department which has the greatest of interest for the general public. This deals with astronomical physics, or astrophysics, as it is sometimes more shortly called; the astronomy, that is, which treats of the constitution and condition of the heavenly bodies, not with their movements. The older astronomy, on the other hand, confined itself to the movements of the heavens so entirely that Bessel, the man whose practical genius revolutionized the science of observation, and whose influence may be traced throughout in Airy's great reconstitution of Greenwich Observatory, denied that anything but the study of the celestial movements had a right to the title of astronomy at all. Hardly more than sixty years ago he wrote: