LIGHT-HOUSES 163 



the employment of lamps of different colours, as in some of the harbour 

 lights which do not require to be seen at a great distance. 



The mode of lighting now generally used in this country is that of 

 placing an argand burner in the focus of a parabolic reflector. This 

 instrument is madeof silver strengthened with copper, and is about three or 

 four inches in focal length, and 21 inches in diameter. The number and 

 arrangement of reflectors in each light-house depend upon the light being 

 fixed or revolving, and upon other circumstances connected with the 

 situation and importance of the light-house. The mode in use in the 

 light-houses of France consists in placing a large argand lamp, having 

 four concentric wicks, and giving a very powerful light in the centre of the 

 upper part of the building, and placing around the lamp a series of glass 

 lenses of a peculiar construction ; thus rising a refracting, instead of a 

 reflecting instrument, to collect the light, and only one lamp instead of a 

 greater number. The lens employed is about 30 inches square, plano- 

 convex, and formed of separate rings or zones, whose common surfaces 

 preserve nearly the same curvature as if they constituted portions of one 

 complete lens, the interior and useless part of the glass being removed. To 

 form a lens of such magnitude out of one piece of glass would be hardly 

 possible, and, if it were possible, the necessary thickness of the glass 

 would greatly obstruct the light ; the merit of the invention consists in 

 building it of separate rings. The light thus obtained is found by 

 experiment to be equal to that afforded by nine common reflectors, and it 

 is calculated that by a consumption of oil equal to that of 17 common 

 argand lamps with reflectors, an effect is produced equal to that of 30 

 lamps and reflectors. There is this further advantage in the French over 

 the English apparatus, that in the English light-house of equal 

 illuminating power with the French, there would be daily employment 

 in trimming 30 lamps and cleaning an equal number of reflectors, which 

 having a very delicate silver surface, require much care and attention, 

 while in the French light-house there is only one lamp to trim, and the 

 lenses being of glass require little or no labour to keep them bright ; on 

 the other hand, these dioptric lights have not the wide dispersive range 

 which is so necessary in fixed lights. On the northern and western 

 coasts of France there are 80 excellent lights, and the Dutch have 20 

 lights on their sea coasts and in the Zuyder Zee. The rates of light duty 

 charged to vessels passing within certain limits vary considerably in 

 respect of different lights : for some of those which are under the 

 management of the Trinity House, as little as a farthing per ton is 

 charged on British, and a half-penny per ton on foreign vessels, while 



