1874, Verplanck Colvin, Annual Report on the Progress of the Topographical Survey of the Adirondack Wilderness of New York, for the year 1873, A helio or sun-reflecting signal was evidently the desideratum.
1925, U.S. Coast and Geodetic Survey, Special Publication - Coast and Geodetic Survey (page 62)A light keeper's duties require him to be alone on a station for days or even weeks at a time, showing a helio to the observer in the afternoon for two or three hours and watching by the signal lamp at night.
1968, Arthur Laidlaw Allan, ‎J. R. Hollwey, ‎J. H. B. Maynes, Practical Field Surveying and Computations (page 258)On a cloudy but bright day, it is possible to use a helio effectively over moderate distances of about 6 miles. Light keepers should be made aware of this, since they often think that brilliant sun is essential.
