232 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



to mention, is that of the apparent light, which was introduced at 

 Stornoway Bay, by Mr. Stevenson in 1852, as shown in Fig. 169, and 

 where, instead of erecting an expensive lighthouse on a sunk rock at 

 sea, a simple perch or beacon was erected, having a lantern on the top 

 containing diverging prisms. A beam of parallel rays thrown from 

 a holophote placed on the shore at the same level as the lantern on 

 the perch, is made after falling on the prisms at the perch to diverge 

 over the required angle of visibility, and in this way the mariner is 



FIG. Itiy. Apparent light. 



led to believe that there is an actual lamp on the beacon, whereas 

 in reality it proceeds from a light on the shore about 650 feet 

 distant. 



During the last few years a fresh innovation has been introduced 

 into light-houses. This is the use of the electric light substituted for 

 that of an ordinary lamp, and consequently there is increase of 

 intensity and range. But the dioptric apparatus remaining the same 

 we need not enlarge on this system here as we shall return to it in 

 the book- devoted tothe applications of electricity. 



