394 POPULAR LECTURES AND ADDRESSES. 



light. Not content with condensing the light to the 

 horizon so as to shed itself out in all horizontal 

 directions, engineers condensed it into certain fixed 

 directions for special reasons. Sometimes they 

 condensed the light into a ray, for the reason of 

 sending it in the direction of a particular channel : 

 sometimes for the sake of giving greater intensity 

 than they could practically attain otherwise, and 

 then they made the ray revolve so as to shed its 

 brightness all round the horizon in the period of its 

 revolution. A policeman's bull's-eye lantern is an 

 instance in point. There is a greater intensity of 

 light in a ray from an ordinary bull's-eye lantern 

 than a light of anything like the same power could 

 give without that optical appliance, or something 

 equivalent to it, or more perfect than it. 



Besides its light, a modern lighthouse generally 

 contains also, for use in such thick or foggy 

 weather that the light cannot be seen, a sound- 

 making appliance, the object of which is not only 

 to be heard, but when heard to be immediately 

 recognised to be itself and nothing else. Mr. Price 

 Edwards, in his communication to the Society of 



