Fig. 387.—Stanley's heliotrope.
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786.—Heliotrope, or heliostat as it is sometimes called, may be any form of mirror to throw the sun's ray in a constant direction or to a distant station at a time of day fixed for making observation. The instrument is uniformly constructed with a small glass mirror having a plane surface. The angle of divergence of the extreme rays in the reflection is the same as that subtended by the sun's diameter at the position of the mirror, that is, of about 32 minutes of arc. This divergence is sufficient to render the reflector visible at a great distance. The plan upon which the author has constructed this instrument is shown in Fig. 387. It consists of a reflector M formed of a plain glass mirror of about 5 or 6 inches in diameter, placed in a metal tray. The mirror is centred vertically upon an axis to which a worm wheel B is attached upon one side that works into a tangent screw which is moved by a milled head so as to place the mirror at any angle to the horizon. The mirror and its vertical adjustment just described are carried by a fork which is erected from the base board of the instrument upon a socket joint which permits the mirror to be turned about. Upon the lower part of the fork above its socket another worm wheel is constructed centrally to the axis. This works into a tangent screw attached by fittings to the base board. The tangent screw has a long shank leading to a milled head A. By means of the milled heads the mirror may be set to any position, so as to throw the reflection of the sun in any required forward direction. A small hole is cut through the silver in the centre of the mirror to sight the position to which the sun's reflection is directed.
787.—The Base Board is of ¾ inch mahogany about 20 inches by 10 inches, and is supported upon a very firm tripod stand, like that described for a plane table, art. 700. At one end of the board a sighting screen of mahogany, 10 inches by 10 inches and ¾ inch thick, is hinged, so as to be held erect by means of a stay bar E. In the centre of the screen an opening is turned out 3½ inches diameter, and a frame-piece of half circle only is placed over this. The frame piece is grooved out at the back so as to hold discs, shown abc in the figure.
788.—The Discs abc, are of thin brass and have openings respectively ¼, ¾, and 1½ inches wide, so as to reduce the width of the line of light which appears through them when the reflection of the sun is thrown from the back. These have each a fine wire stretched across them to indicate the centre. A fourth disc, not shown, has a double cross of wires to indicate the centre only.
789.—To Pack the Instrument, the screen is turned down the index frame, falling into the opening F; the mirror with its fork is lifted out and secured to the surface of the base board by buttons; and the whole apparatus is put in a pine case. Its weight without tripod stand is 8 lbs.
790.—To Use the Heliotrope, the station on which the sun's light is to be thrown is sighted by looking through the small hole in the centre of the mirror, and adjusting the base board until the station appears in the centre space of the disc opening. The mirror is then turned towards the sun by means of the milled heads until its image, reflected upon the back of the screen, appears central with one of the discs which is intended to be used. All parts of the stand and fittings being made quite firm, the attendant moves the milled heads, as required, to follow the apparent motion of the sun, at intervals of five minutes or less. It must be observed that the centre of the slit in the disc represents the station visible to the observer. This point must therefore be plumbed to the station point in setting up the instrument. A part of the screen at P is cut away to admit of the suspension of a plummet.
791.—The heliotrope was much used in India for the great trigonometrical survey. Colonel H. Thuillier states from experiment that "A heliotrope of 9 inches diameter answers for 90 to 100 miles. For nearer distances it is much too bright to be observed through a telescope, and the light must be diminished in the following proportion. For distances of 2 or 3 miles (the usual distance of a referring mark) an aperture of 0·25 of an inch will answer, and for longer distances about 0·1 of an inch of aperture per mile of distance will suffice, viz., an inch for 10 miles, 2 inches for 20 miles, and so on, provided always the apparatus is carefully adjusted and the man who works is alert and skilful."[56]
Practically the discs here described will give all the variation required. In less favoured climates than India more opacity will be found in the atmosphere, and larger apertures required than those just stated.