325 



SECTION M EC II A NICS. 



part of the light, we can at once avail ourselves of the spherical mirror ; 

 but the light that is passing above that would still be lost. In order 

 to intercept that, there is suspended above the flame a half holophote, 

 which has the property of combining the rays which fall upon it in a 

 parallel beam. Then, above this vertical beam of parallel rays there are 

 prisms which were first used at Buddon Ness right angled conoidal 

 prisms, which also have the property of spreading the light which falls 

 upon them over forty-five degrees. In this way, by the union of the 

 following instruments, the whole light coming from the flame is com- 

 pressed into the arc of forty-five degress, viz., Fresnel's lens, Fresnel's 

 fixed light prisms, the azimuthal condensing prisms, the cylindric re- 

 fractor, the dioptric spherical mirror prisms, the holophote, and the 

 conoidal prisms above. 



The next thing I shall allude to is a simple apparatus for illuminating 

 sunken rocks at sea. At Stornoway, in the Island of Lewis, in 1851, 

 it was proposed to place a lighthouse upon a sunken reef, which 

 would have been a very serious matter. Instead of that, a main 

 light was placed upon shore, and in order to point out the position of 

 the reef at night, the following plan was adopted. A perch or 

 beacon, which is comparatively an inexpensive structure, was erected 

 upon this sunken rock and on the top of the perch there was a lantern 

 containing certain optical prisms. Exactly on a level with that lantern 

 there was a window cut in the tower of the lighthouse, and in 



r igure II. 



