202 LECTURES TO SCIENCE TEACHERS. 



In those cases, when certain arcs in azimuth require less 

 light than others, a part of the light thrown upon them 

 might be usefully spent upon the remainder of the circum- 

 ference of light. The apparatus invented to meet these 

 various requirements, and which I am about to describe, 

 have reached such a pitch of perfection, that it may be 

 almost laid down that all the light from a lighthouse lamp 

 can be projected in whatever directions the constructor may 

 desire. 



Before entering on a description of the apparatus I shall 

 at once state that all the information upon these subjects, 

 as far as I am aware, is to be found in the works of Messrs. 

 Alan and Thomas Stevenson, and in that of M. Reynand, 

 of the French Ponts et Chaussees Department. These 

 works are Mr. Alan Stevenson's description of the Skerry- 

 core Lighthouse, and one called " Lighthouse Illumination," 

 by one of the family. M. Reynand's is on the lighting 

 and buoying of the coasts of France. 



Messrs. Stevenson, following in Fresnel's footsteps, have 

 reached by their inventions, coupled with his, the extra- 

 ordinary perfection to which I have alluded. All the cal- 

 culations of the forms of Fresnel's catadioptric rings, and 

 of Mr. T. Stevenson's new forms of prisms, will be found 

 in the works I have alluded to, and I shall only describe 

 the models before you, their principle and object, with the 

 aid of a few diagrams (enlarged from Messrs. Stevenson's 

 works), without attempting to enter into their calculation, 

 which would supply by itself materials for a course of 

 lectures. 



The first historic lighthouse may be said to be that of 

 Corduan on the Garonne in France, erected nearly 300 

 years ago, and about 200 feet h-'gh, first lighted by a wood 

 fire, then in 1784 by Lenoir's lamps and paraboloidal re- 

 flectors, and in 1822 by Fresnel s dioptric instruments. 

 Then come in order other historical structures, such as 

 Smeaton's Eddystone Lighthouse, twice destroyed by fire, 

 and the Messrs. Stevenson's Bell Rock and Skerry core 

 lighthouse, the description of t.he erection of this last by 

 Mr. Alan Stevenson himself being one of the most in- 

 teresting records of ability and perseverance in the annals 

 of civil engineering. It was for this Skerrycore light that 

 the first great improvements in Fresnel's apparatus were 



