The visibility of distant objects.

For thousands of years, thousands of mariners have sighted thousands of ships, and have made appropriate entries in their logs. Even so, this mass of miscellaneous information is of little use in predicting the range at which a specified object will be just visible under a new set of circumstances. The purpose of this paper is to identify the principal factors involved in the visibility of an object, to indicate how each factor affects the range of visibility, and to supply charts which, by combining these factors, enable the limiting range to be found under any set of prevailing conditions. (This paragraph has been lifted, almost verbatim, from some material prepared during the war by Professor Arthur C. Hardy, then Chief of the Camouflage Section (16.3) of the NDRC.)