MECHANICAL AND USEFUL ARTS. 87 



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°, 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°, 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 flr.me of an argand lamp |ths 

 of an inch wide, and lh inches high, be placed in the focus of an 

 ordinary Trinity-house parabolic reflector, it will supply a beam 

 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 3§ 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 neces- 

 sity 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. Now 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 money it can be raised to a five or tenfold power, or 

 more, and will then give five or tenfold effect. This can be done, 

 not merely without increase of the volume of the light, but whilst 

 the light shall have a volume scarcely the 2000th part of that of the 

 oil flame. Hence the extraordinary assistance we may expect to 

 obtain of diminishing the size of the optical apparatus and perfecting 

 that part of the apparatus. 



Many compressed intense lights have been submitted to the 

 Trinity-house ; and that corporation has shown its great desire to 

 advance all such objects and improve the lighting of the coast, by 

 spending, upon various occasions, much money and much time for 



