228 THE 'APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



machinery is arranged in the lantern which surmounts the tower, 

 and also of the several architectural arrangements of the build- 

 ing itself. The lighthouse at Cordouan has quite a monumental 

 aspect, being entirely constructed of stone. That of New Caledonia, 

 quite recently erected, is on the contrary of sheet-iron and cast-iron. 

 Tt was made in Paris, then taken to the place of its destination, 

 where it has been for eight years. 



In 1849, Mr. Thomas Stevenson, of Edinburgh, suggested what he 

 has termed the holophotal arrangement, which is applicable alike to 

 the catoptric and dioptric systems of lighthouse apparatus. In order 

 to save the light which escapes uselessly past the lips of the para- 

 bolic reflector, the following combination shown in Fig. 162 is resorted 

 to : a is the paraboloid (in this case truncated at its parameter), L is a 



FIG. 10'2. Holophotal arrangement. 



lens which, at its focal distance for parallel rays, subtends the same 

 angle from the flame as the outer lips of the paraboloid, so that each 

 ray passing outwards is thus intercepted and parallelized either by 

 the lens or the paraboloid. The other half of the rays passing back- 

 wards, falls upon the hemispherical mirror I I (which takes the place 

 of the apex of the paraboloid) and these rays are reflected back again 

 by its action through the flame so as after again diverging, to be 

 parallelized by the agency of the lens and paraboloid in front. The 

 flame is therefore at once in the centre of the spherical mirror, and 

 in the common focus of the lens and paraboloid. 



The above arrangement, though theoretically perfect, is objection- 

 able, on account of the great loss of light which always takes place 

 by absorption wherever metallic reflection is employed. In order to 



