LIGHTHOUSE ILLUMINATION. 205 



upper and lower portions (of which we will now speak) not 

 being exactly directed towards the same arc in azimuth 

 with the central drum. In the splendid light exhibited in 

 the grounds outside this collection, constructed by Messrs. 

 Chance Brothers for the Little Basses lighthouse, Ceylon, 

 another arrangement of the lenses is adopted. It is like 

 one of these complete lenses cut in two, and the two 

 halves given a slight angle, so that there is a double 

 flash. 



To utilise the light passing above and below these 

 lenses, Fresnel invented what are called catadioptric rings 

 or zones. These he only used for small lights of what are 

 called the fourth order, and it remained for Mr. A. Stevenson 

 to apply the same principle to the larger lights. Before 

 proceeding with this I shall say one word with regard to 

 the manner in which catadioptric rings are formed. There 

 is in every transparent substance what is called a critical 

 angle. If a ray passing through one face of a prism, meets 

 another face at an angle less than the critical angle of the 

 material, that ray is totally reflected, and no refraction 

 takes place. 



The paths of the two"extreme rays for each prism really 

 determine its form. Of course everything connected with 

 the direction of these rays depends upon the index of re- 

 fraction of these various glasses, of which flint glass is the 

 highest. I should mention that the curves there, instead 

 of being the true curves, found by formulae, are always made 

 circular, and the other sides are either straight, or one 

 slightly concave, and the other convex. The ray passes on to 

 meet the prism, and a similar calculation gives its form. The 

 prisms you see in these beehive -formed lights are con- 

 structed on this system, and the lower prisms are similarly 

 calculated. With this arrangement all the light is utilised, 

 and sent out in as many parallel groups of rays as there 

 are sides to the polygonal lantern or beehive. When only 

 a fixed light is required, which is to be seen from all points 

 of the compass, the polygonal form would not answer, as 

 the arcs in azimuth near the angles would be in darkness. 

 For this purpose Fresnel adopted a circular lantern, the 

 lenses of prisms being generated by the same section, but 

 revolving round a vertical axis passing through the centre 

 of the lamp. Here is one of that form of the fourth order 



