one have emphasized the conditions involved in a critical study of the surface features of Mars. In the experience of an astronomer, it is not an unusual occurrence that an object in the heavens, fairly conspicuous, remains unseen until by some lucky chance an observer sweeping the sky picks it up, and, having determined its position, it is promptly found by others. Professor H. H. Turner, in his "Astronomical Discovery of the Nineteenth Century," says: "It is a common experience in astronomy that an observer may fail to notice in a general scrutiny, some phenomenon which he can see perfectly well when his attention is called to it; when a man has made a discovery, and others are told what to look for, they often see it so easily that they are filled with amazement and chagrin that they never saw it before." In the Rev. T. W. Webb's interesting book on "Celestial Objects for Common Telescopes," a reminiscence of the author is given by a friend in which the following is related as illustrating the varying ability of observers in seeing. "A curious instance of difference of vision was well illustrated one superb evening when Mr. Webb and the writer were observing Saturn with the nine and a half inch refractor at Hardwick. Mr. Webb saw distinctly the division in the outer ring which the writer could not see a trace of, while the writer picked up a faint point of light which afterwards turned out to be Enceladus (a satellite) which Mr. Webb could not see." In my brief observation of Mars I probably might have made out many more details if I had permitted Mr. Lowell to tell me what to see, and where to look for them on the disk. This I would not allow him to do, nor did I study any of the numerous drawings in his own work, or the original memoirs of Schiaparelli, or other works containing drawings of Mars in his library. I would not learn the names of any of the regions, or canals, nor with a single exception do I know them now. Only when I had finished my last night's observations, did Mr. Lowell take my drawings and write out a list of the various canals, oases, etc., which I had made out. Thus, unaided, I drew simply what was plainly evident, though many other details flashed out for a second, which were not recorded, simply because I did not see them often enough to be sure of their precise position on the disk.