The first essential of the signal station is visibility, the second being that of concealment from hostile observation. In acquiring a mean between conflicting requirements, the following special considerations in the selection of stations should be considered.
_Backgrounds._--Backgrounds are important factors in the selection of signaling sites.
Sky backgrounds are desirable as affording strong contrast and are therefore conducive to celerity in the transmission of signals. They are rare and can only be secured when stations are located on the exact crest of ridges, on mountain peaks, or on lands which bound the horizon of view from the other stations. Stations with sky backgrounds, while affording the best facilities for transmission, are little adapted to the requirement of secrecy.
Dark backgrounds are far more common and more easily obtainable than sky exposures. They afford the maximum means of concealment from hostile observation, but materially reduce the range, speed, and accuracy of signal transmission.
Mixed or broken backgrounds are those which display varied colors behind the signals. Backgrounds of this description do not accord with either of the essential requirements of the signal station and should be avoided whenever possible.
In general, sky backgrounds should always be selected for signal stations when conditions are such that the requirement of secrecy can be dispensed with; if, on the other hand, there is reason to fear that the signals may be intercepted by the enemy, dark backgrounds should invariably be chosen, even though the disadvantages they impose, render them less desirable visually.
_Azimuth of stations._--The azimuth of signal stations should, if possible, be such that the visual lines of information should intersect the vertical plane through the apparent course of the sun, at a considerable angle. Stations located so as to be unavoidably viewed from these directions during portions of the day are very liable to appear enveloped in a haze, and telescopes, if turned upon them, are filled with dazzling light. If the location of stations on or close to the sun line is unavoidable, sites affording sky exposures should be chosen. Exposures of this kind obviate to a great extent the difficulty of sun haze and should be secured when this difficulty is encountered and it is impracticable to change the azimuth of the station.