FOR GREA T BRITAIN AND IRELAND. \ 1 1 



When alteration of course is necessary after passing a 

 danger, two more lights in one, of a different colour, should 

 be provided for steering on the fresh line of bearing. 

 Sometimes a screened light is used showing only over a 

 sector of a certain number of degrees, to keep boats or 

 vessels on a safe bearing. 



The two following works may be consulted with 

 advantage : ' Rudimentary Treatise on Lighthouses,' by 

 Alan Stevenson, LL.B. : Weale, London, 1850; and ' Light- 

 house Illumination,' by Mr. T. Stevenson: Weale, 

 London, 1859. 



"THE MOCK OR APPARENT LIGHT 



is a simple means of illuminating a sunk rock or other 

 danger which is inaccessible in stormy weather. This 

 object is attained without requiring a lamp or any kind 

 of flame to be placed on the rock itself. A pole or 

 beacon, carrying on its top a lantern containing different 

 forms of optical agents (depending on the nature of local 

 requirements), is all that is needed on the rock itself. A 

 beam of parallel rays, proceeding from a distant lamp 

 placed on the shore, is thrown seawards upon the lantern 

 on the beacon, where the incident rays are re-dispersed by 

 the apparatus, so as to produce an optical deception to the 

 eye of the mariner, who supposes he sees a lamp burning 

 where there is really none. The apparent light is also 

 very useful for marking the seaward ends of breakwaters 

 or piers. In entering harbours at night, sometimes one 

 pier-head must be hugged, sometimes the other, according 

 to the direction of the wind ; and so critical in stormy 

 weather is the taking of a harbour, that even a single yard 

 of distance may be of consequence. Those only who know 



