ON LIGHTHOUSE CHARACTERISTICS. 397 



a short time a fraction of a second, or from that 

 to five or six seconds and then disappears ; and, 

 for a much longer time than the duration of the 

 flash, it remains invisible, until it again flashes out 

 as before. In the fixed light there is no dis- 

 tinguishing characteristic whatever, but merely a 

 light seen shining continuously and uniformly, 

 The occulting light is visible during the greater 

 part of its time like a fixed light, shining continu- 

 ously and uniformly. Characteristic distinction is 

 given by a short eclipse, or by a very rapid group 

 of two or three short eclipses, or of short and 

 longer eclipses recurring at regular periods, " flashes 

 of darkness," as they have been called, cutting out, 

 as it were from the light its mark, by which it may 

 be distinguished and recognised to be itself and 

 nothing else, in the very short time (from half- 

 second at the least, to seven seconds at the most) 

 occupied by the group of eclipses. 



I. FLASHING LIGHTS. 



Six years ago, in every flashing light there was 

 just one flash in the period, and thus the length of the 



