CHAP. II.] 



LIGHTHOUSES. 



231 



of this apparatus in which 1*93 of spare light on the landward side 

 is allocated by the lens B and prisms a, and the lens c and prisms b, 

 so as to strengthen the light passing down the Sound from the front 



FIG. 167. Arrangement of the prisms. 



apparatus over the arc a and up the Sound over the arc 0. By 

 apparatus of a similar kind the whole light from the flame has been 

 condensed into an arc of 45 at the 

 lights of the Tay, and of 30 at Cape 

 Maria Van Diemen, in New Zealand. 



Another lighthouse optical agent 

 which was introduced in Scotland in 

 1869 at Lochindall Lighthouse, is 

 that shown in section in Fig. 168, 

 where it will be noticed that the 

 principle of single-acting prisms has 

 been extended to embrace very large 

 angles behind the flame. These prisms 

 were first proposed by Mr. Stevenson 



and Mr. A. Brebner, and independently by Professor Swan of St. 

 Andrews. 



The only other lighthouse arrangement which it seems necessary 



FIG. 168.-Lens atthe Lochiudall Lighthouse. 



