2d, corroboration and extension by Professor Pickering of his discoveries at Arequipa with regard to the forms of Jupiter’s Satellites;[10]
3d, the discovery and study by Mr. Douglass of the atmospheric causes upon which good seeing depends.
It is of the observations connected with the first of these that the present volume of the Annals alone treats.
As the publication of this volume has been so long delayed, it seems fitting to add here a brief continuation of the history of the observatory to the present time.
The results of the expedition in 1894, in the detection of planetary detail, turned out to be so important an advance upon what had previously been accomplished that the writer decided to form of the temporary expedition a permanent observatory. Accordingly, he had Alvan Clark & Sons make him a twenty-four-inch refractor, which fate decided should be their last large glass; the Yerkes glass, although not yet in operation at the time this goes to press, having been finished at nearly the time his was begun. The glass received from Mantois happened to be singularly flawless and its working the same. It was made twenty-four inches in clear aperture, and of a focal length of thirty-one feet. Alvan G. Clark accompanied the writer to Flagstaff and put the glass in place himself.
The mounting for the telescope was likewise made by the Clarks. Rigidity was the prime essential, in order to secure as stable an image as possible, and this has been admirably carried out, the mounting being the heaviest and most stable for a glass of its size yet made.
In July, 1896, Dr. T. J. J. See joined the observatory, to continue there the line of research for which he was already well known—the study of the double stars. This added to the two initial objects of the observatory a third,—
3d, the study of double-star systems, including a complete catalogue of those in the southern heavens.
During the summer and autumn of 1896 the importance of good atmosphere was further demonstrated in an interesting and somewhat surprising quarter. The air by day was found to be as practicable as that by night. While Mars was being studied by night, the study of Venus and Mercury was taken up during the daytime systematically, and the results proved as significant as had been those on Mars. Instead of the vague diffused patches hitherto commonly recorded, both planets’ surfaces turned out to be diversified by markings of so distinct a character as not only to disclose their rotation periods but to furnish the fundamental facts of the physical conditions of their surfaces. We know now more about Mercury and Venus than we previously knew of Mars.
As the winter in Flagstaff is not so good as the summer, it was thought well to try Mexico during that season of the year. Accordingly, a new dome was made; the telescope was taken down; and dome, mounting, and glasses were carried to Mexico and set up for the winter at Tacubaya, a suburb of the City of Mexico, at an elevation of 7500 feet. There the observatory received every kindness at the hands of the President, the Government, and the National Observatory.