LIGHTHOUSE ILLUMINATION. 09 



spherical, but of increased radius ; this is what gives the 

 irregular appearance to the back of the mirror. By the 

 construction of all these apparatus the light from the lamp 

 is all utilised for illuminating an arc in azimuth of 82. 



The last of these lights I have to describe is the fixed 

 azimuth condensing light for Buddonness at the entrance 

 of the Tay, to illuminate an arc of 45. It is in the collec- 

 tion, but the model is too large for transport. I have here 

 vertical and horizontal sections showing the lenses and 

 prisms employed. There are nowhere above four refractors 

 and four total reflections of a ray. Five optical agents are 

 employed in the construction. As an example of the 

 utility of this I will give you a rough plan of the place it 

 was intended for, namely, the Isle of Oronsay near the 



; A 



f 1234 SXttct 



FIG. 4. 



Isle of Skye, where the light is utilised for guiding ships 

 along two channels. I must now hurry on to say one 

 word about a highly important part of illumination in light- 

 houses. You have often buoys and beacons placed in har- 

 bours, some of them almost unapproachable, and where it 

 may be difficult or impossible to erect a lighthouse ; hence 

 what are called dipping lights have been employed. A 

 lighthouse on the adjacent coast contains not only its own 

 summit light, but also a light from a window, as repre- 

 sented, dipping towards the direction of the reef or rock, 

 and of such divergence that when a ship comes within 

 range of it, mariners will know that they must change 

 their course. Sometimes in place of a window ligkt, part 



VOL. II. 



