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STANDS FOR INDIRECT-VISION REFLECTORS,

THE ALT-AZIMUTH STAND.

Fig. 19.

The word Alt-azimuth is a contraction of Altitude and Azimuth. These terms refer to the two movements which must be given to a telescope mounted on this plan, to enable the apparent motion of celestial objects, due to the revolution of the earth, to be followed, and thus retained in the field of view for continuous observation. When a star is due south, or, as it is technically called, “ on the meridian,” it has attained its greatest “ altitude,” or height, from the horizon. The motion preceding this meridian passage is, from the rising of the star—an ascending one—and that immediately succeeding â– —a descending one—till the star has set. To follow this ascending and descending motion, the telescope must therefore have a Vertical or Altitude Movement, which is thus applied:—The Telescope being balanced on trunnions, can be moved from an elevation approaching the Zenith to a horizontal position. In order that it may be secured anywhere between these extremes, attached to the upper part of the telescope is an iron rod, which, sliding through the end of the arm of the stand, can be there clamped. The telescope will now be clamped in Altitude. As the progression of celestial objects will apparently be very slow, resource must be had to the smoothness of motion obtained by a screw. The upper end of the Altitude rod is therefore tapped to receive a long screw with a large milled head, jointed to the telescope body ; by revolving this head, the telescope is raised or depressed accordingly as the screw is unscrewed, or the reverse. It is necessary that the screw should be withdrawn some way from the rod before clamping it, preparatory to following an object which has passed the meridian, or is setting; as, perhaps, just when the clearest vision is obtained, the observer may be annoyed by the screw action being suddenly stopped by the milled head coming in contact with the top of the rod.

When viewing objects near the zenith, the handle attached to the clamp will be found useful, as it can thereby be reached without leaving the finder. The handle may be so placed that a downward push should clamp, and an upward pull release.

We have lately introduced a great improvement on this handle, substituting for it a long rod shaped as an eccentric, and passing through

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