I860.] the Electric Light. 221 



the ray passes between the air and the glass ; and also in some degree 

 by absorption in tlie 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 obser- 

 vation 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^ shall be compressed into a beam diverging only 15°, th&t it may 

 give in the distance a bright flash having a certain duration instead of 

 a continuous light, — or into one diverging only S** or 6°, 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 given size. If the flame 

 of an argand lamp -^th of an inch wide, and H inches high, be placed 

 in the focus of an ordinary Trinity-house parabolic reflector, it will 

 supply abeam having about 15° divergence : if we wish to increase 

 the effect of brightness, we cannot properly do it by enlarging the lamp 

 flame ; for though lamps are made for the dioptric arrangement of 

 Fresnel, which have as many as four wicks, flames 3i inches wide, and 

 burn like intense furnaces, yet if one be put into the lamp-place of 

 the reflector referred to, its effect would chiefly be to give a beam 

 of wider divergence : and if to correct this, the reflector were made 

 with a greater focal distance, then it must be altogether of a much 

 larger size. The same general result occurs with the dioptric 

 apparatus ; and here, where the four-wicked lamps are used, they are 

 placed at times nearly 40 inches distant from the lens, occasioning the 

 necessity of a very large, though very fine, glass apparatus. 



On the other hand, if the light could be compressed, the necessity 

 for such large apparatus would cease, and it might be reduced from 

 the size of a room to the size of a hat : and here it is that we seek in 

 the electric spark, and such like concentrated sources of light, for aid 

 in illumination. It is very true, that by adding lamp to lamp, each 

 with its reflector, upon one face or direction, power can be gained ; 

 and in some of the revolving lights, ten lamps and reflectors unite to 

 give the required flash. But then not more than three of these 

 faces can be placed in the whole circle ; and if a fixed light be required 

 in all directions round the lighthouse nothing better has been yet 

 established than the four-wicked Fresnel lamp in the centre of its 

 dioptric and catadioptric apparatus. Kow the electric light can be 

 raised up easily to an equality with the oil lamp, and if then substituted 

 for the latter, will give all the effect of the latter ; or by expenditure of 



