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Page:Popular Science Monthly Volume 57.djvu/22

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tion to it if an observer possesses a perfectly plane mirror, which it is very difficult and also expensive to obtain; if the reflecting mirror should be imperfect it would distort the image of the corona. The second method, lacking a good mirror, is to mount the long-focus lens in a tube and point it directly at the sun. A forty-foot lens was thus mounted at Cape Ledo for December 22, 1889, and its action was very satisfactory. Of course, it was a cumbersome arrangement, and could not be employed by a small party. The foundation for the mounting of the forty-foot tube consisted of two casks filled with stones and cement, and set firmly in the ground. These made two good piers, since the narrowing tops of the casks held the bed plates of the telescope as in a vise. A triangle, whose base was parallel to the earth's axis and having the telescope tube itself for the long side, was fitted with an extension rod for adjustment in altitude on the third side, and the whole was made to revolve on ball bearings. This triangular support was rotated by a side rod of adjustable length, whose end terminated in a sand piston working with a regulating valve. The sand flowed out steadily like an hour-glass, and dropped the tube, keeping it central on the sun. The image was made to follow accurately for twenty minutes without tremor, all the time holding the solar disk tangent to fixed lines. The principle of a revolving triangle and a short piston, taking the place of an expensive reflecting mirror with a delicate clockwork or one carrying a telescope balanced on its center but subject to jars and side motions, is an important assistance in field work on account of its ready adaptability to all sorts of observations. Since time is limited, it is necessary to provide all operations with automatic arrangements as far as possible, by using such an apparatus as that described. What can be applied successfully to a forty-foot lens can serve for shorter telescopes. In combination with spectroscopes, polariscopes, and special instruments for photographing, an immense amount of work can be compressed into the few seconds allowed by mounting them all on such a movable frame as the triangle. The old-fashioned method of putting one observer to one telescope ought to be abandoned. Of course, for a rising sun during the forenoon a modification in the moving support must be employed. This should be such as to cause the objective of the telescope to rise from the ground toward the meridian, and it must be accomplished by attaching a heavy weight which in sinking draws the tube upward.

Of the two kinds of photographic plates, the wet and the dry, the dry plates are much more convenient in the field, and are good for certain purposes. The objections to them are that the rough granulations of the gelatin film sometimes overpower the fine de-