| | | eclipsed by a group of two | | | | or more eclipses. | +--------------------+---------+-------------------------------------+ In the foregoing classifications only a white light is used. But it may so happen that the lighthouse, owing to its position and the dangerous character of the spot which it marks, carries a light which changes colour from white to red or green, which are shown alternately in various combinations. These characteristics are indicated as follows: +--------------------+------------+----------------------------------+ | Type of Light. | Symbol. | Characteristics. | +--------------------+------------+----------------------------------+ | | | | | Alternating | Alt. | White and colour alternating. | | | | | | Alternating | Alt.Fl. | Flashing alternations by | | flashing | | revolving mechanism. | | | | | | Alternating fixed | Alt.F.Fl. | Fixed and flashing alternating. | | and flashing | | | | | | | | Alternating group | Alt.Gp.Fl. | Group flashing alternating. | | flashing | | | | | | | | Alternating | Alt.Occ. | Occulting alternately with | | occulting | | white and coloured | +--------------------+------------+----------------------------------+ In timing a revolving or flashing light, the cycle is taken from the beginning of one flash to the beginning of the next. In these readings the flash is always shorter than the duration of the eclipse, while an occultation is shorter than, or equal to, the length of the light interval. Since flashing and occulting may be carried out with a fixed light suddenly extinguished or eclipsed, the characterization is determined solely according to the relative duration of light and darkness, irrespective of the type of apparatus employed or the relative brilliancy. There is one peculiarity of the flashing light which may be remarked. At short distances and in clear weather a faint continuous light may be shown.