The coronas of the past forty years are shown on Chart III, taken from the report of the eclipse of 1896 (August 9th), by A. Hansky. It arranges the coronas in the eleven-year period so far as the dates at which the eclipses occurred permit this to be done, and by comparing them in vertical lines the similarity is at once seen for the respective quarters of phases of the period. The forecast there given for 1900 is seen to resemble 1867, 1878, and 1889, but it differs in orientation from that on Chart IV, which was prepared by the author. The four coronas on the left in Chart III are taken at the sun-spot maximum, and the appearance is that of total confusion in the structure of the rays; the second and the fourth columns are for the sun’s medium intensity at about halfway between the maximum and the minimum, and they show a system of polar rays taking on structural form, the second column being at a stage of diminishing and the fourth at one of increasing solar activity; the third column gives the corona when the spots are at a minimum of frequency and the sun is in a comparatively quiescent state, wherein the polar rifts are very distinct and the equatorial wings or extensions greatly developed. [Illustration: CHART IV.--BIGELOW’S FORECAST OF THE CORONA OF MAY 28, 1900. E, earth’s axis; K, axis of ecliptic; S, axis of sun; C, C, poles of the solar corona.] The successful observation of a solar corona depends upon three conditions: the selection of the instrument, its proper mounting, and the photographic process, regarding each of which a few suggestions will be made. The instruments are divided into two classes, for visual and for photographic work. But in either case the most important feature is the focal length or the size of the telescope. Since the photographic image of the corona will not bear magnifying without dispersing the available light, and thus blurring out the details of the picture, which is the most important feature to retain to the utmost, one can not use a short telescope and at the same time a magnifying eyepiece to enlarge the image by projection on a screen or on a photographic plate. The only alternative in order to get an image of large diameter is to use a long-focus lens. The effect of a difference of focus upon the image of the corona is well shown on Chart V, which gives a small corona (1) taken with a four-foot lens (Barnard), (2) with a fifteen-foot lens (Pickering), and (3) with a forty-foot lens (Schaeberle). The diameter is proportional to the focal length, but the difference of effect upon the details is very important. In the small picture the details of the corona near the sun are completely lost in the general light, while the coronal extensions from the middle latitudes are seen at a great distance from the sun--as much as one million miles; at the same time the polar rifts are distinctly marked, so that the pole or central line from which they bend is readily located.