OBJECT OP THE LIGHT-HOUSE. 119 



seen. But the purpose of the light from a light-house is 

 not to enable the observer to see any thing except itself, 

 but to see itself only for the purpose of enabling him to 

 determine where he is. It does not make visible to him 

 the entrance into the harbor, nor show him the rocks and 

 shoals which he is to avoid, but only to show itself, and, by 

 so doing, to mark a point, for the purpose simply of mak- 

 ing known to the mariner where he is. Being guided in 

 this way only in determining his position, he must depend 

 upon his chart, or his own knowledge of the coast lines 

 near, for his guidance into the entrance of his haven. 



Thus it happens that, for a beacon on the shore of the 

 sea, there is required, not a diffused, but a highly concen- 

 trated light, to show itself to the mariner simply as a star 

 beaming from the midst of surrounding darkness. All that 

 the mariner requires of it is that it should show itself to 

 him. He does not expect that it will reveal to him any 

 of the surrounding objects. These he must have in his 

 memory, or in the mental conceptions which he forms from 

 his chart. The light is only to enable him to place himself 

 properly among them. 



There is another thing that is remarkable and is very 

 important to be understood in respect to such a light, and 

 that is, that it is only that portion of it which shines in 

 certain limited directions that is useful for the purpose re- 

 quired namely, that which goes forward over the sea 

 and of this only that portion which passes along at a mod- 

 erate distance above the surface of the water. The light 

 from any luminous point radiates naturally, as has already 

 been explained, in every direction, so as to illuminate a 

 complete sphere. A very large portion of this sphere is 

 cut off, of course, by the ground. Half of it would be so 

 cut off if the light was at the surface of the ground and 

 the ground was level ; but, as the light is raised above the 



