NATURAL PHILOSOPHY. 109 



English ph} T sicist, and the result of his researches has been the discovery of 

 a ready and simple means of applying the electric discharge from the induc- 

 tion coil to the purposes of illumination. A carbonic acid vacuum tube 

 (that is, a tube filled with carbonic acid, which is then exhausted from it by 

 means of an air-pump, until there is only the most infinitesimal trace of 

 gas remaining), having an internal diameter of about one-sixteenth of an 

 inch, is wound in the form of a flattened spiral; to the ends of the tube are 

 attached two wider tubes into which platinum wires are sealed; they are 

 inclosed in a wooden case, so as to permit only the spiral to be exposed. 

 When the discharge from a Ruhmkorff's induction apparatus is passed 

 through the vacuum tube, the spiral becomes intensely luminous, exhibiting 

 a brilliant white light. M. Gassiot, who exhibited the instrument in action 

 at a recent meeting of the Royal Society, caused the discharge to pass 

 through two miles of copper wire, showing that it would be applicable to 

 illumination at a distance. The results were brilliant in the extreme; and 

 it was confidently predicted that the new device would shortly constitute 

 one of the most useful and popular fprms of the electric light. 



ON THE USE OF THE ELECTRIC LIGHT FOR LIGHTHOUSE ILLUMI- 

 NATION. 



The following is an abstract of a lecture on the above subject by Professor 

 Faraday, recently delivered before the Royal Institution, London : 



The use of light to guide the mariner as he approaches land, or passes 

 through intricate channels, has, wilh the advance of society and its ever in- 

 creasing interests, caused such a necessity for means more and more perfect, 

 as to tax to the utmost the powers both of the philosopher and the practical 

 man, in the development of the principles concerned, and their efficient 

 application. Formerly the means were simple enough; and if the light of a 

 lantern or torch was not sufficient to point out a position, a fire had to be 

 made in its place. As the system became developed, it soon appeared that 

 power could be obtained, not merely by increasing the light, but by directing 

 the issuing rays ; and this was, in many cases, a more powerful and useful 

 means than enlarging the combustion, leading to the diminution of the 

 volume of the former, with, at the same time, an increase in its intensity. 

 Direction was obtained, either by the use of lenses dependent altogether upon 

 refraction, or of reflectors dependent upon metallic reflection ; and some an- 

 cient specimens of both were shown. In modem times the principle of 

 total reflection has also been employed, which involves the use of glass, and 

 depends both upon refraction and reflection. In all these appliances nmch 

 light is lost. If metal be used for reflection, a certain proportion is ab- 

 sorbed by the face of the metal; if glass be used for refraction, light is 

 lost at ah 1 the surfaces where the ray passes between the air and the glass; 

 and also in some degree by absorption in the body of the glass itself. There 

 is, of course, no power of actually increasing the whole amount of light, by 

 any optical arrangement associated with it. 



The light which issues forth into space must have a certain amount of 

 divergence. The divergence in the vertical direction must be enough to cover 

 the sea from the horizon to within a certain moderate distance from the 

 shore, so that all ships within that distance may have a view of their lumi- 

 nous guide. If it have less, it may escape observation where it ought to be 

 seen; if it have more, light is thrown away which ought to be directed within 



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