September 29, 1892] 



NATURE 



515 



to increase the diameter of the apparatus. In 1883 

 Messrs. Stevenson got an offer from Messrs. Barbier for 

 a lens of 1840 mm. focal distance. 



All refracting lenses from the day of Alan Stevenson 

 were cylindrical for fixed lights and plano-convex for re- 

 volving lights, and no alteration of any moment has been 

 made in the mode of their construction until 1888, when, 

 instead of making the lenses cylindrical or plano-convex, 

 I proposed to give them a spherical form, that is to say, 

 circular not only in the horizontal but also in the vertical 

 section. This design was carried into practice in the 

 apparatus for one of the Fair Isle Lighthouses. The in- 

 troduction of the spherical refractor has made practicable 

 the construction of very much larger and consequently 

 more powerful apparatus, and occupying much less space 

 both in the daylight size and diameter of lantern, and, 

 hence, diameter of tower. It has rendered practicable 

 the quadrilateral arrangement with hyper-radiant lenses 

 which have already been erected at Fair Isle, the lenses 

 being cut so as to give two flashes from each side of the 

 quadrilateral. An experimental lens made for Mr. Wig- 

 ham is to be tried in Ireland. It is 2m. focal distance, 

 and the spherical refractor is 7 feet 6 inches diameter, 

 and will give one flash from each side of the quadrilateral. 

 M. Barbier says that in making this lens he was attempt- 

 ing to give the most powerful flash possible, and he 

 adopted the spherical refractor. In this case, however, 

 the spherical refractor has been carried, in my opinion, 

 rather far, except in the view of economy in keeping the 

 angles of the whole apparatus within reasonable limits, 

 which is only possible (in an apparatus of 4m. diameter) 

 by the use of the spherical refractor, and by its being 

 made to subtend a great angle. When I proposed this 

 form I pointed out that there was a loss of efficiency if it 

 subtended more than 40' ; now M. Barbier has made the 

 spherical portion subtend 64'', or 24° farther, and 8" 

 farther than any spherical refractor yet made. The 

 great amount of light which I experimentally found re- 



by the greater divergence which takes place /« the spherical 

 refractor, and which would be a small source of loss in 

 a revolving light, but would better illuminate the nearer 

 sea in a fixed arc. 



Equiangular Refractor. 

 To obviate the loss of light at the outer face of the 

 lenses, especially those remote from the focal plane — a 



F/G.2. 



turned from the inner face of the spherical refractor made 

 for Fair Isle, however, shows that up to 20°, and perhaps 

 farther, it is ample to make up for any loss of light caused 



f O 1 196, VOL. 46] 



loss which stops the refractor being carried with due 

 regard to efficiency farther than 30° to 40° in the 

 cylindrical form, and 20° in the spherical form — it will 

 be found that the most efficient form is a refractor which 

 I proposed, with the inner face of each lens doing equal 

 work with the outer face, or nearly so— and by a careful 

 study of such a refractor it will be found that the locus of 

 the centres of curvature of each refracting lens lies out- 

 side the refractor, and at points below the focal plane, 

 and more and more remote from the lens, as the lenses 

 are more and more remote from the focal plane, and that 

 the inner face of the refractor ought, in fact, to be a 

 parabolic curve (Fig. i). This can very closely be 

 approximated to by a circular curve with a suitably 

 chosen centre on the focal line produced, but the centre 

 is so far distant that when the spherical form and the 

 equiangular form are used in combination (Fig. 2), the 

 inner face of the equiangular prisms above and below it 

 may, with sufficient accuracy, be made straight and lean- 

 ing outwards in place of being vertical as in Fresnel's 

 form. By the combination of the spherical refractor and 

 the equiangular, or a refractor of the equiangular form 

 alone, the defect in Fresnel's refractor, namely the loss of 

 light at emergence from the lenses, especially those remote 

 from the focal plane, is avoided, and the refractor may 

 thus be made to subtend an angle which has hitherto 

 been considered inexpedient, with glass of the ordinary 



