2S5 



ON LIGHTHOUSE ILLUMINATION— THE ELECTRIC LIGHT. 



BY PROFESSOR FARADAY, D.C.L., F.R.S. 



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

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

 increasing 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 lanthorn or torch was not sufficient to point out a position, a 

 fire had to be made in their 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 depen- 

 dent altogether upon refraction, or of refractors dependent upon metallic 

 reflection ; and some ancient specimens of both were shown. In modern 

 times the principle of total reflection has also been enrployed, which 

 involves the use of glass, and depends both upon refraction and reflection. 

 In all these appliances much light is lost : if metal be used for reflection, a 

 certain proportion is absorbed by the face of the metal ; if glass be used 

 for refraction, light is lost at all 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 

 luminous 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 the useful degree of divergence : or if the horizontal 

 divergence be considered, it may be necessary so to construct the optical 

 apparatus, that the light within an angle of 60 or 45 deg. shall be 

 compressed into a beam diverging only 15 deg., that it may give in the 

 distance a bright flash having a certain duration instead of a continuous 

 light, — or into one diverging only 5 or 6 deg., which, though of far 

 shorter duration, has greatly increased intensity and penetrating power in 

 hazy weather. The amount of divergence depends in a large degree upon 

 the bulk of the source of light, and cannot be made less than a certain 

 amount, with a flame of a ^ Wen size. If the flame of an Argand lamp §th of 

 an inch wide, and 1| inches high, be placed in the focus of an ordinary 

 Trinity-house parabolic reflector, it will supply a beam having about 15 deg. 



