110 ANNUAL OF SCIENTIFIC DISCOVERY. 



the tiscful decree of divergence; or if the horizontal divergence be considcrcrl, 

 it may be necessary so to construct the optical apparatus, that the light 

 within an angle of sixty or forty-five degrees shall be compressed into a 

 beam diverging only fifteen degrees, that it may give in the distance a bright 

 flash having a certain duration instead of a continuous light; or into one 

 diverging only five or six degrees, 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, seven-eighths of an inch wide 

 and one and a half inches high, be placed in the focus of an ordinary Trinity 

 House parabolic reflector, it will supply a beam having about fifteen degrees 

 divergence. If we wish to increase the effect of brightness, we cannot prop- 

 erly 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 

 three and a half 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 

 forty 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 direc- 

 1ion, 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 estab- 

 lished 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 ten-fold effect. This can 

 be done not merely Avithout increase of the volume of the light, but whilst 

 the light shall have a volume scai-cely the two-thousandth, 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 it. 



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 this end. It is manifest that the use of a 

 lighthouse must be never-failing, its service ever sure; and that the latter 

 cannot be interfered with by the introduction of any plan, or proposition, or 

 apparatus, which has not been developed to the fullest possible extent, as TO 

 the amount of light produced, the expense of such light, the wear and tear 

 of the apparatus employed, the steadiness of the light for sixteen hours, irs 

 liability to extinction, the amount of necessary night care, the number of 



