56 



NA TURE 



[November 21, 1895 



system of colour nomenclature might be devised by indicating 

 colours in a manner analogous to the accepted nomenclature of 

 the points of the compass. 



In this connection the enclosed circular, published by me dur- 

 ing the summer of 1893, to explain briefly my system of colour 

 standards, may be of interest to your readers. You will see that 

 the system of colour nomemclature here described corresponds 

 almost precisely to the idea in Mr. Spencer's mind. 



October 24. Louis Prang. 



[We have received, with Mr. Prang's letter, the circular descrip- 

 tive of his system, which is essentially the same as that suggested 

 by Mr. Spencer. As Mr. Spencer pointed out in his letter to 

 us, the idea is a very obvious one, and had probably occurred to 

 others ; but this does not, of course, diminish its value. For 

 ourselves, we are glad to see that the idea has been put into 

 practice, and that, out of a chaos of colour-names, an intelligent 

 system of nomenclature has been evolved. A series of standard 

 coloured papers, harmoniously bound together, is published by the 

 Prang Educational Company, Boston, for the use of teachers, 

 designers, artists, and others. — Ed. Nature.] 



RECENT IMPROVEMENTS IN LIGHTHOUSE 

 ILLUMINATION. 



PRIOR to the year 1822, the optical apparatus used 

 in the best equipped lighthouses was silver-plated 

 parabolic reflectors, having apertures, twenty-one inches 

 for fixed, and twenty-five inches for revolving lights, their 

 focal distance being four inches. These instruments, 

 with a burner one inch diameter, and an initial power 

 of eleven candles, condensed the light into a beam of 

 1304 and 2360 candles respectively. The power of a 

 first order reflector light of sixteen reflectors, ranged on a 

 frame having four faces, was equal to 9340 candles. 

 These were the instruments (Fig. i) which A. Fresnel, 



the celebrated physicist, replaced by optical apparatus of 

 glass lenses and prisms acting by refraction and reflec- 

 tion in combination with a central lamp and multiple- 

 wick burner of considerable power. In doing so he 

 devised four optical agents : the annular lens, totally 

 reflecting prisms, the cylindric refractor for small ap- 

 paratus, and straight vertical reflecting prisms. These 

 instruments he combined in different ways to form fixed 

 and revolving apparatus suitable for lighthouse purposes. 

 The annular lens subtended at the focus 45° vertically 

 and horizontally, the focal distance being 920 m.m. (36-22 

 inches). Some of Fresnel's optical instruments in the 

 hands of Mr. Alan Stevenson, underwent very notable 

 alteration, extension, and improvement. The first order 

 lens was extended to subtend 57° vertically, and reflect- 

 ing prismatic rings were substituted below the lenses for 



NO. 1360, VOL. 53] 



silvered mirrors. The central portion of the first order 

 fixed light apparatus (Fig. 2) was converted from a polygon 

 formed of narrow lenses to a cylindrical drum, which 

 drum was divided into sections having helical joints, and 

 above and below the central belt reflecting prisms were 

 introduced. 



The next improvements were those of 1849-1850, by 

 Mr. Thomas Stevenson, the main feature of which was- 

 that, for revolving lights, totally reflecting prisms gener- 

 ated round a horizontal axis were introduced above and 

 below the lenses, lenticular action being thus extended 

 throughout the whole height of the instrument (Fig. 3). 



Then followed the dioptric spherical mirror, the diop- 

 tric holophote, the azimuthal condensing system, and 

 its application to fixed, flashing, intermittent, and groups 

 flashing lights ; the bivalve apparatus of 1859^ which is 

 really two holophotes ten feet in diameter, increased in 

 power by a dioptric mirror, and numerous combinations, 

 of these and previously devised instrunaents. 



