208 



LECTUJRES TO SCIENCE TEACHERS. 



the prism c c. This system of prisms employed by Fresnel 

 for flashes may be used for increasing the divergence and 

 intensity of light in certain arcs of azimuth. 



We now come to what is called condensing apparatus, 

 which consists in utilising the useless rays which would 

 otherwise fall on the land, or on some parts of the horizon 

 where they are not wanted. This apparatus which I shall 

 now describe is a fourth order condensing apparatus con- 

 structed for the Lamlash lighthouse. Fig. 3 is a vertical 

 section of the prisms and lenses of the Lochindaal con- 

 densing apparatus, and applies equally to the Lamlash 

 model. ABC is a half beehive on Fresnel's system. In 

 this case the angle in azimuth to be strengthened is of 82. 



as > 



FIG. 



The rays from the central Fresnel beehive over the arc b c 

 are diverted by vertical prisms shown in section at 1 to 16, 

 and, to take up less space, some of them, namely 10 to 14, 

 are placed behind the others, and room is made between the 

 front prisms for their rays to pass by ; using twin prisms 

 such as from 4 to 9 to give more space. The rays from 

 the whole semicircle, abc, are thus utilised on the arc ab ; 

 for the most divergent rays of prisms, 1 and 14, are nearly 

 parallel to the radial lines fa, fb. The half not represented 

 of the circle abc is completed by a dioptric spherical mir- 

 ror, which sends back all the rays striking the focus /. 



It will be remarked that the central zones of these 

 mirrors are close together, the upper and lower are also 



