ON LIGHTHOUSE APPARATUS. 



327 



that a holophote was placed, and a powerful beam of parallel rays 

 was projected upon the lantern in the perch. The optical prisms 

 redistributed the rays which fell upon them, and to a seaman 

 coming into the harbour the light appears to come from the 

 perch, whereas in reality it comes from the shore about 530 feet 

 distant. This kind of light is used in several other places. (Shown 

 in fig. 11). The only other matter which I think it is necessary 

 to mention is some new forms of prisms which have been recently 

 introduced at Islay. (Shown in fig. 12). They have the remark- 



Figure 12. 



a, b, c, lens and hoiophote ; a, g, c, h, new back prisms ; i, j, k, dioptric spherical 



able property, of parallelizing rays, for an incidence as low as 

 about 1 80 degrees. I think it right to state that these prisms 

 were invented independently by Mr. Brebner and myself, and by 

 my friend Professor Swan of St. Andrews, who gave the formula 

 for constructing them. Another new form of prism, (fig. 13), which 

 was described in Nature some months ago, is called a twin prism. 

 This prism has its apex cut out, the object being not only to 

 carry out an ingenious suggestion of Professor Swan of passing the 

 light from a set of prisms through the chinks in another set placed 

 before them, but to reduce the thickness of the glass through which 

 the light has to pass. In this way you save much light, which 

 would otherwise be lost by absorption. The first example of this 

 kind of apparatus is now being made by Barbier and Fonestre of 

 Paris, for the Scotch Lighthouse Board. 



