ON LIGHTHOUSE APPARATUS. 



323 



continued to be made at Paris and elsewhere in which two agents were 

 employed. Nor did Mr. A. Stevenson, who spent about a month 

 at the Lighthouse Works in Paris, in 1834, see or hear anything of 

 those prisms. Again, in Leonor Fresnel's description of apparatus 

 which he published in 1844, he represents some improvements made 

 in his brother's light, but he still employs two agents. I may also 

 state that in 1850 I went over to Paris with my late brother, Mr. 

 Alan Stevenson, in order to explain by means of a drawing and 

 model this, among other inventions, but at that time M. Fresnel 

 did not say anything about this old model; and still more striking 

 is the fact that M. Degrand, who succeeded Fresnel as. one of the 

 engineers of the Lighthouse Board in Paris, claimed these prisms as 

 an invention of his own in 1851. And equally inexplicable is the fact 

 that M. Tabouret, who is said to have made the model of 1825, 

 exhibited a new revolving light at the Great Exhibition of 1851, in 

 which no such prisms were used. The only apparatus at the Exhibi- 

 tion in which they were, was sent by the Scotch Board. Lastly, so late 

 as 1851 a first order revolving light was constructed for Cape L'Ailly 

 in France. It was originally ordered on the yth of May, 1851, by the 

 French Administration of the old construction in which there were no 

 prisms of the kind to which I have referred, but subsequently that order 

 was recalled and the light was made holophotal, after the French 

 engineers* had seen the holophotal apparatus which was then being 

 made for the Scotch Lighthouse Board, by M. Letourneau. 



I shall now pass on to the azimuthal condensing principle for 

 lighthouses. There are many peculiar situations which the lighthouse 



Figure 7. 



