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 motion. As fast as the star leaves the field, therefore, a touch of the instrument will bring it into view again. Obviously, however, if the star's apparent motion were very great, it would take most of the observer's attention to keep it in the field. Any refined observation would thus be rendered impossible. To remedy this difficulty, clock work is attached to the instrument. This is so arranged that the motion of the telescope is exactly equal and opposite to that of the earth in rotation. The observer is thus enabled to study and measure at his leisure, without a thought but that instrument and star are alike stationary. When equipped with the best of mounting and the most perfect of clock work, with stable foundations and adequate protection from the weather, the telescope might seem complete. But it is poised thirteen feet above the floor. The eye piece has a sweep of ninety degrees and is far out of reach most of the time. Without some means of getting to it, the instrument would be practically useless. It was for Professor Bond to meet this difficulty, and he devised the observing chair which is still in use. In appearance it is rather a formidable looking piece of machinery, but it is simple in use. By its means the observer can convey himself easily and rapidly to any desired part of the dome. It may be well for us to remind ourselves that an astronomer would consider it a waste of time simply to sit and look through the tube of the telescope. Unless he wants a half hour's amusement, he will attach to it one of the little instruments which are shown us, for measuring minute distances and angles, or for measuring or analyzing light. The Harvard telescope is much used for measuring
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