A twenty-one inch reflector allows the rays issuing from it to diverge fifteen degrees. So that we have the light of the 360 degrees (the whole of the circle) gathered into fifteen (a twenty-fourth part of the circle). It does not quite follow that within that area the light will be twenty-four times as strong as if allowed to dissipate itself all round, because something must be allowed for absorption and waste; but we believe this allowance has been greatly overstated, and that where there are no mechanical difficulties in the way, the reflecting system is decidedly the best. Of course where it is necessary to light more than fifteen degrees of the circle, it will be necessary to use more reflectors, placing them side by side round a shaft, and if these are set into revolving motion, focus after focus of each reflector comes before the eye of the mariner, and the effect is all that can be desired.
The dioptric or refracting system of lighting is the reverse of this. In the reflector the light is caught into a basin and thrown out again. In the refracting system, in its passage through the glass prisms, it is bent up or down and falls full upon the eye of the mariner, instead of wasting itself among the stars or down among the rocks at the lighthouse foot. For light, falling upon glass at a certain angle, does not go straight on, but gets deflected and transmitted in an altered line, as it does through water. And here comes the weakness of the dioptric system, in close vicinity to its strength. It is true that prisms and lenses send the light in the direction which is desired, but they charge a toll for the transmission; the glass is thick, and somewhat of the nature of a sponge. If we write on blotting-paper the marks appear on the other side, but some of the ink has soaked sideways, and there is very little doubt, that when light is transmitted through glass, a good deal of it is absorbed and retained.
To those who have never seen a dioptric apparatus, or a diagram of one, it would be very difficult to make any written description intelligible. The reader must imagine a central lamp, with three or four circular wicks, making up a core of light four inches across, and as many high. Round this, and on a level with it, at a distance of three feet from it, go belts of glass. From these belts, or panels, the light goes straight out to sea, but as there is a great quantity of light which goes up to the ceiling and down to the floor, rings of prisms are put above and below the main panels, and these catch the upper and lower light, and bend it out to sea, parallel to the main central beam. When a revolving light has to be made by the dioptric apparatus, the lenses are so constructed that the light, in going through them, is gathered up into the exact similitude of a ray, as it would leave the mouth of a reflector, and of course with the same result; the central lamp remains stationary, and the lenses move round it, and focus after focus, flash after flash, come upon the eye of the mariner. Both the systems admit of peculiar adaptations, and they have been occasionally combined into a hybrid apparatus, called Cata-dioptric.
This, then, is, in brief, the history of the development of the lighthouse system of the United Kingdom. We think it has fairly kept pace with the development of the mercantile marine. When there were coal fires at some lighthouses, and wax candles at others, the vessels that passed them, and were guided by them, were not such as modern shipwrights would look at with much complacency. But the age that has produced the Great Eastern can also point to the Skerryvore and the Bishop Rock lighthouses, to the Electric light, and to a first-class Lighting apparatus.
Whatever the future in store for the English lighthouse system, one thing is certain, that it cannot, and was never meant to supersede seamanship. No amount of lighting will dispense with the necessity for eyes, no extent of warning is so good as the capacity for grappling with a danger. The lighthouse authorities may cheer a sailor on his way with leading lights and beacon warnings, but he must still be a sailor to turn their warnings to account.
When Rudyerd’s Eddystone was building, Louis XIV. was at war with England, and a French privateer took the men at work upon the rock, and carried them to a French prison. When that monarch heard of it, he immediately ordered them to be released, and the captors to be put in their place; declaring that, though at enmity with England, he was not at war with mankind. Extremes meet. The most gorgeous monarch in Europe and the plain Quaker of Liverpool had one point in common;—they both agreed that the erection of a Lighthouse was “a great holy good, to serve and save humanity.”
BESSY’S SPECTACLES.