At its return in 1758 the prediction erred on the side of thirty-two days; at the return in 1835, by a margin of only two days; and in 1910, by the amount of two and one-half days. Perchance, ere it makes its next appearance in 1985,[10] the presence of another planet beyond Neptune may have been detected, explaining the disturbing factor resulting in that small discrepancy. The astronomers at that remote date (1984) may succeed, therefore, in making a prediction so exact that the comet may “swim into their ken” promptly to scheduled time. Few, if any, of the present-time readers of this book (unless it falls into the hands of a very youthful enthusiast) will be here to welcome the comet at its next return, and even the youthful enthusiast may have the distressing experience of the American astronomer Dr. Lewis Swift, who saw Halley’s comet in 1835, and was able to welcome it at its return in 1910, but, owing to failing eyesight, was unable to see it, much to his regret. [Illustration: The orbit of Halley’s Comet, which it passes over in 75 to 77 years, showing where the comet is to be found now, and during its course until its next return in 1985. ] With regard to my first visit to the Yerkes Observatory, the following facts regarding the great refractor may be of interest, as well as the incident narrated to me by Miss Calvert while we were awaiting Professor Barnard’s invitation to look for Halley’s comet, on that momentous occasion. The story was deferred, in my account, to the final part of this chapter, so as not to break the thread of the actual account of my first view of the comet. Following the description of the telescope, the story of a catastrophe which nearly ended its career is best told in Professor Barnard’s own words, as quoted from the after-dinner speech, in January, 1907, at Nashville, already referred to in this book. “The tube of this instrument is about sixty-four feet long. In the farther end of this tube is placed the great object glass, forty inches clear in aperture. When one is looking overhead with this giant telescope, he must be at a point some thirty feet or more lower than when the tube is pointed toward the horizon. To avoid the use of a high ladder to reach the observing end of the telescope in its various positions, the floor of the dome itself is made into a giant elevator, sixty-five feet in diameter. The rising and lowering of this floor—which is done by electric motors—always keeps the observer in a convenient and safe position with reference to the eye-end. This floor is suspended by heavy steel cables which go over wheels at the tops of four towers attached to the inside walls of the dome. The floor is counterpoised by heavy iron weights at the other ends of the cables.