210 LECTURES TO SCIENCE TEACHERS. 



of the light of the upper lantern is collected by lenses 

 below it, and reflected out in the required direction. If 

 there is danger of confusion between the two lights, the 

 upper one can be white, the reef light red. When the 

 land is low and the danger distant the dipping light would 

 be insufficient for protection, and another course has been 

 resorted to, namely, that of apparent lights. One of these 

 has been erected at the entrance of Stornoway Bay with 

 perfect success. The beacon is rarely accessible. Its light 

 is thrown from a window of a lighthouse in a strong hori- 

 zontal beam on a -mirror on the beacon, from wlr'ch it is 



.. HI mm 



FIJ. 5. 



reflected through a lens so as to diverge in the required 

 direction for ships entering Stornoway Bay. 



The silver of the mirror gets injured by damp, but after 

 attempting hermetically sealed lanterns it was found neces- 

 sary to unseal them as the sea air got in through the putty, 

 and caused a haze over the glasswork. Mr. Stevenson's 

 new prisms have been used for the same purpose, but in 

 that case this prism serves instead of the mirror and lens. 



In spite of all these improvements there appears little 

 doubt that a good reflector, with a holophote such as I 

 described before, gives excellent results. Mr. Stevenson 

 reconmends that the metal section of the reflector shall 

 be parabolic, the horizontal section hyperbolic, or elliptic, 



