It was soon found impracticable to revolve an optical apparatus with its mountings, sometimes weighing as much as 7 tons, at the higher rate of speed required for feux-eclairs by means of the old system of roller carriages, though for a certain number of small quick-revolving lights ball bearings have been successfully adopted. It has therefore become almost the universal practice to carry the rotating portions of the apparatus upon a mercury float. This application of mercury rotation was the invention of 0. Bourdelles. The arrangement consists of an annular cast-iron trough containing mercury with a similar but slightly smaller annular float immersed in it and displacing a volume of the liquid metal whose weight is equal to that of the apparatus supported. In all cases provision is made for lowering the mercury bath or raising the float and apparatus for examination. An example of a mercury float is shown in Plate II., fig.3.
In order to double the power to be obtained from a single apparatus at stations where lights of exceptionally high intensity are desired, the expedient of superimposing one complete lens apparatus on another has some times been adopted, as at the Bishop rock (fig. 4), Hartland point (Plate II.) and at the Fastnet lighthouse in Ireland (Plate II.). Triform and quadriform apparatus have also been erected in Ire land. The adoption of the multiform system involves the use of lanterns of increased height. Another method of doubling the power of a light is by mounting two complete and distinct optics side by side on the same revolving table. This expedient has been frequently adopted by French designers.
Augustin Fresnel divided his diop tric lenses into "orders" or sizes depending on their focal distance. This division is still used, although two additional "orders" known as "small third order" and "hyper-radial" respectively are in ordinary use. The following table gives the focal distance of the several sizes :— Lenses of smaller focal distance are also made for buoy and beacon lights.
The powers of lighthouse lights given in the Admiralty Lists of Lights are expressed in terms of "light house units" (one lighthouse unit = i,000 pentane candles). In France the "bougie decimale" and in America the "American candle" are identical with the standard pentane candle. The Hefner unit used in Germany, Holland, etc., is 0.9 of the pentane candle. The intensity of a beam of light emitted from a dioptric apparatus depends upon the brilliancy or intrinsic brightness of the source of light and the projected area of the optical panel, in the case of a revolving light, or of the pillar of light in the case of an optic of fixed section. The theoretical value of the intensity can be com puted from the formula 1=iA. Where 1 is the total intensity of the emergent beam; i is the mean intrinsic brightness obtained by dividing the total light intensity of the source by the area of its projected image in various planes and A the projected area of the optical panel (for revolving apparatus) or the height of glass multiplied by the breadth of the illuminating source (for optics of fixed section).
In practice, owing to the inequalities of the illuminating source and the losses due to refraction, reflection and absorption expe rienced by the rays in their passage through the optical elements and the lantern glazing, the actual value is found to be about 5o% of the theoretical value. By the addition of a mirror the intensity of that part of the panel reinforced by it is increased by about 3o%. Deductions must be made in the case of coloured lights (see paragraph on "coloured lights," p. 92).
The mathematical theory of optical apparatus for lighthouses and formulae for the calculations of profiles will be found in the works of the Stevensons, Chance, Allard, Reynaud, Hopkinson, Ribiere and others.