2.30 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK. in. 



other optical agents in front. No light therefore reaches the eye of 

 an observer placed behind the apparatus, though between him and the 

 flame the screen is of transparent glass. These prisms of double 

 agency have lately been improved by Mr. J. T. Chance. 



In lights of the first order where there is but one great central 

 burner, as in Fresnel's revolving light, Fig. 156, the light passing- 

 above the lens instead of being intercepted 

 as in his arrangement by the double agency 

 of inclined lenses and mirrors, is at once 

 parallelized by the single agency of the 

 holophotal prisms as shown in Fig. 165. 

 The application of total reflection to re- 

 volving apparatus was first employed at 

 the Horsburgh lighthouse in 1850. 



In particular cases, depending upon the 

 physical peculiarities of the locality, sucli 

 as narrow seas and sounds, the whole light 

 must be spread horizontally with strict 

 equality over some one given arc, or in a 

 light of unequal range, where it must be 

 seen at different distances in different 

 azimuths, the light must be allocated to each of such arcs in 

 the compound ratio of the number of degrees and the distance from 



FIG. 1(55. Stevenson's revolvi 

 light. 



FIG. 166. Application of azimuthal condensing prisms 



which the light requires to be seen. Fig. 166 is a chart show- 

 ing Isle Oronsay in the Sound of Skye, on the west coast of Scot- 

 land, which was one of the three of those azimuthal-condensing 

 lights which were first lighted in 1857. Fig. 167, represents a plan 



