Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . ight by a mirror from the station sighted uponto that occupied by the observer. The beam of reflectedlight is pointed upon as on a signal. There are three specificobjects to be aimed at in the design and use of the heliotrope: 1. The reflecting surface should be as near the center ofthe station as possible; 2. The method of aligning or directing the reflected beamtoward the observers station should be the most precise andsimple attainable; 3. The method of
Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . ight by a mirror from the station sighted uponto that occupied by the observer. The beam of reflectedlight is pointed upon as on a signal. There are three specificobjects to be aimed at in the design and use of the heliotrope: 1. The reflecting surface should be as near the center ofthe station as possible; 2. The method of aligning or directing the reflected beamtoward the observers station should be the most precise andsimple attainable; 3. The method of maintaining the direction of the re-flected beam, while following the apparent movement of thesun, should be the simplest possible. There are three general types of heliotropes for theaccomplishment of the above objects. These are : 1. Simple hand-mirrors provided with screw for attach-ment to a wooden support; 2. Telescopes carrying revolving mirror and aligningsights; 3. Steinheil heliotrope having mirror aligned by the re-flected image of the sun. Heliotropes should rarely be used as signals for distances OBSERVING SCAFFOLDS, 567. Fig. 165.—Observing Scaffold and Signal. DIMENSIONS OF HELIOTROPES. 509 less than twenty miles, excepting for very smoky or hazyweather, or because of the difficulty of making visible anopaque signal in dense wood ; or at any greater distance pro-viding an opaque signal can be seen. This chiefly because (i)opaque signal gives better definition; (2) the beam reflectedfrom the heliotrope is too large when observed at short range;(3) it is difficult to arrange a satisfactory understandingbetween the observer and the distant heliotroper. The dimensions of a heliotrope should be the smallestwhich will produce a clearly defined and visible star of lightat the distance observed. In order, therefore, to secureimages of uniform size at all distances, the size of the mirrormust be varied according to the distance. For ordinaryatmospheric conditions and di
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