ON LIGHTHOUSE APPARATUS. 



be seen down stairs. (Shown in fig. 9 in section, and in fig. 10 in 

 plan). The object there, was to condense the whole of the light 



Vertical Section. Horizontal Section. 



Figs. 9 and 10. 



a, b, c Fresnel's fixed light, p straight condensing prisms, i. h suspended portion of holophote, 

 , /conoidal prisms. 



coming from the flame into an arc of forty-five degrees in azimuth. 

 The first thing to be done was to intercept the front half of the light 

 by 108. of Fresnel's fixed light apparatus. But all that is in this case 

 required to be illuminated is forty-five degrees. Fresnel's apparatus 

 answers the purpose perfectly for the forty-five degrees, but what be- 

 comes of the light which comes from the other part of his fixed 

 apparatus ? In order to condense that light into the required directions 

 there are straight prisms which were first used at Isle Oronsay in 

 1857, which straight prisms stand in front of the apparatus, as will be 

 seen down stairs, and all the light (less forty-five degrees) which 

 passes out of Fresnel's fixed light is received upon those straight 

 condensing prisms, and they spread the light over the forty-five 

 degrees, so that the whole of 0113 half of the light is condensed into 

 an arc of forty-five degrees ; that is to say, the light which passes 

 through the sides is so bent by refraction and total reflection as to 

 reinforce that which comes from the lorty-five degrees in front. 

 Of course those straight prisms do not reflect the light in the 

 vertical plane, but only in the horizontal. With regard to the back 



