Fig. 31.
Plan of Observatory (lower floor).
This tripod support in connection with the sustaining of the telescope by the wire rope, gives that steadiness which is so essential in photography. Only a slight amount of force, about two pounds, is required to move the instrument in azimuth, though it weighs almost a thousand pounds.
The plan of the frame centrally carried by the axis a is as follows: From the corners of a parallelogram i i (2 × 13 feet) of wooden beams, eight inches thick and three inches broad, perpendiculars n n′, Fig. [28], rise. At the top they are connected by lighter pieces to form a parallelogram, similar to that below, and just large enough to contain the tube of the telescope. At right angles to the parallelogram below, and close upon it, a braced bar o o′, Fig. [28], crosses. From its extremities four slanting braces as at p p′, Fig. [28], go to the corners of the upper parallelogram, and combine to give it lateral support. At the top of one close pair of the perpendiculars n′, Fig. [28], are bronze frames carrying friction rollers upon which the trunnions move, while similarly upon the other pair n are two pulleys, also on friction rollers, for the wire rope coming from the counterpoises.
Movement in altitude is very easily accomplished, and with the left hand upon the winch i, under high powers, both altitude and azimuth motions are controlled, and the right hand left free. The whole apparatus works so well, that in ordinary observation the want of a clock movement has not been felt. Of course for photography that is essential.
§3. THE CLOCK MOVEMENT.
The apparatus for following celestial bodies is divided into two parts; a. The Sliding Plate-holder; and b. The Clepsydra. In addition a short description of the Sun-Camera, c, is necessary.
a. The Sliding Plate-holder.
Mr. De La Rue, who has done so much for celestial photography, was the first to suggest photographing the moon on a sensitive plate, carried by a frame moving in the apparent direction of her path. He never, however, applied an automatic driving mechanism, but was eventually led to use a clock which caused the whole telescope to revolve upon a polar axis, and thus compensate for the rotation of the earth, and on certain occasions for the motion of the moon herself. In this way he has produced the best results that have been obtained in Europe. Lord Rosse, too, employed a similar sliding plate-holder, but provided with clock-work to move it at an appropriate rate. I have not been able as yet to procure any precise account of either of these instruments.