As we ascend we notice that the walls of the chamber as far up as the gallery are formed of iron plates, while above that there is a metal framework filled in with glass panes, and above all a dome-shaped roof.
Having reached the platform we proceed to examine the glass, and we find that the metal framework forms a cage with four sides, each approximately flat, but really slightly spherical. Each of these sides is called a "panel." In the centre of each is a lens. Peeping through the interstices between the prisms, we perceive that the lamp is inside this structure, exactly in the centre, so that its light shines directly through the central lens or bull's eye. Around this bull's-eye are many circles of glass bar, forming refracting prisms. Around this again are more bars in the form of segments, which together form circles, some being refracting prisms and others reflecting prisms. All the light rays from the lamp which fall on any one prism are deflected, so that they proceed approximately in the same direction. Those prisms in the upper part lay hold of the rays which would otherwise go up into the sky. Those at the bottom collect those which would fall near the foot of the tower. So scarcely any are lost. But for the fact that the lamp itself is comparatively large and not a theoretical point, as already explained, the beam from this panel would be perfectly straight, parallel, and of uniform density everywhere. As it is, it widens slightly as it proceeds, but, practically speaking, we might call it a solid beam of light.
Each of the panels sends forth such a beam, so that they strike out in four directions from the central lamp much as four spokes from the hub of a wheel.
Then descending once more to the floor from which we started, we see that among the columns there is a large clockwork arrangement, the purpose of which is to drive round the turntable and all that it carries—in the language of the lighthouse engineer the "optical apparatus" or, more briefly, "the apparatus." And as this turns the radiating beams of light sweep round the horizon and in succession strike into the eyes of any mariner who may be within range. Each time a beam strikes him he sees a flash. If the apparatus revolve once a minute he will see four flashes every minute, one from each panel.
Let us consider, then, the advantages of this wonderful mechanism, with its cunning arrangement of prisms. It is these latter, of course, which are the important thing. The rest, the mechanical portion, is simply for the purpose of holding them and turning them at the proper speed. In the first place, the contrivance gives us flashes instead of a steady light; it gives the lighthouse its "character." Then again it enhances the brightness of the light. Instead of shining all round, the light is concentrated in four special directions, and the light which would be wasted upwards or downwards is saved and brought into use.
But suppose that the lighthouse we are considering be near the shore, so that there is no need for it to throw any light in one—the landward—direction. Then we should see inside the revolving framework with its prisms a fixed frame with reflecting prisms which would catch any rays going from the lamp in the direction of the land and simply hurl them, as it were, back into the flame. Thus the intensity of the flame becomes increased by those rays thrown back which would else have been wasted.
Or suppose that the character of the light is such that the flashes have to be at irregular intervals. Then the framework, instead of being symmetrically four-sided, would be of an irregular shape.
And that brings us to a beautiful feature of the mechanism of the apparatus. We have been discussing a four-panel arrangement. Suppose that we were to reduce it to three. Then, since all the light would be concentrated into three beams instead of four, each beam would be more intense. We should thereby have increased the range of our apparatus without any increase in the cost of oil—for nothing, as it were. But to get the same number of flashes per minute we should have to drive it round so much the faster. But increased speed means increased burden on the keepers who have to wind up the heavy weights which operate the clockwork. So there is a limit to the speed which can be attained.
But if friction can be almost eliminated the apparatus can revolve at a high speed without throwing undue burden upon the men. But how can friction thus be got rid of? Messrs Chance Bros., the great lighthouse constructors, of Birmingham, have done it, almost entirely, by floating the apparatus on mercury. The turntable has on its under side a large ring which nearly fits a cast-iron trough on the top of the pedestal. In this trough there is mercury, so that upon the liquid metal the apparatus floats as if upon a circular raft. The table with its lenses, prisms and other fittings may weigh six or seven tons, yet it can be pushed round by one finger.
The various sizes of optical apparatus are known as "orders." One of the "first order" has a focal distance of 920 millimetres. This means that there is that distance between the centre of the lamp and the bull's-eye. They descend by successive stages down to the sixth order, with a focal distance of 150 millimetres, while the most important lights are of an order superior even to the so-called "first," termed the "hyper-radial," the focal distance of which is 1330 millimetres.