We see, then, that the effect which a fog produces is mainly to refract the light rays. Each little drop of water (for it must be remembered that fog is myriads of tiny drops of liquid; it is not vapour) acts like a minute lens, and bends the rays which pass through it. And the more blue a ray is the more it is bent. On the contrary, the more red it is the less is it bent. When a beam of light is analysed in the spectroscope the red rays are bent least and the blue rays most, so that the red rays fall at one end of the spectrum and the blue at the other.
Now we only see a thing when light rays proceeding from every part of it fall straight (or nearly so) upon our eyes. Consequently, since red rays are bent and scattered by the fog less than blue rays are, a red light will be more easily seen through a fog than a blue one. It might seem from this that a red glass put in front of a light would make it better for this purpose, but that is not the case, for the simple reason that filtering the light through red glass does not really make it any redder than it was before: it simply makes it look redder by extracting from the original light all except the red. But a source of light which is naturally reddish is so because it is more plentifully endowed with red rays, while a bluish light like the electric arc is naturally deficient in red rays. Consequently we should be inclined to expect from theory that the electric arc would not be a good light for a lighthouse, since it would lack penetrating power in foggy weather. Some readers may have noticed themselves, in towns where electric lights and gas lamps are in use near each other, that the latter, though relatively feebler under normal conditions, seem to give more light in fog. And experiments show that this is really the case. So although there are some lighthouses with electric arc lights, that which is now believed to be the best is an oil lamp of special design, using a mantle of the Welsbach type.
The oil is stored in strong steel reservoirs into which air is pumped by means of a pump not unlike those used to inflate bicycle tyres. By this means a pressure is maintained upon the oil of about 65 lb. per square inch. This forces the oil up a pipe and drives it in a jet into a vaporiser, a tube heated from the outside so that in it the oil is turned into gas. This gas then rises to the burner and heats the mantle, just as the gas does in the ordinary incandescent gas light. Indeed in the case of lights on the mainland near a town the gas from the town main is often utilised. But this simple arrangement for using vaporised oil, as will readily be seen, can be employed anywhere. A little of the gas produced is led through a branch pipe and burnt to heat the vaporiser. To start the apparatus the vaporiser is heated with a little methylated spirit. Thus everything is quite self-contained and so simple that there is little to get out of order. The largest size of lamp will give 2400 candle-power, with an expenditure of 2 1⁄4 pints of oil per hour, just common oil, too, of the kind used with ordinary wick lamps.
Having got a source of powerful light, the next thing is to collect that light and throw it in the direction required. For the light proceeds from the lamp in all directions (practically), and much of it would be entirely wasted could it not be collected and guided in the required direction.
The earliest attempt at this was to use a reflector of bright polished metal. In the most improved form these were made to that peculiar curve known as a parabola. This is a curve obtained by cutting a cone in a certain way, wherefore it is one of the "conic sections," and its particular appropriateness for this work resides in the fact that if a light be placed at a certain point known as the "focus" all the diverging rays which fall upon the reflector will be reflected in the same direction, parallel to each other. An ordinary spherical mirror would reflect them either back to the lamp or in diverging directions.
At any distance the beam from the parabolic reflector will be more intense than that from the spherical one, since the rays will be closer together. But even with the parabolic one there is some diffusion, for the simple reason that whereas the focus is a mathematical point (position without magnitude) the most concentrated form of light known has a considerable magnitude. Hence the rays proceeding from the centre of the mantle are reflected as per the theory, but those from the outlying parts of it are somewhat diffused. This difficulty cannot possibly be overcome, and hence even in the finest examples of lighthouse architecture the flashes are not quite sharp and clear-cut. There is a central moment, so to speak wherein the flash is almost blinding in its intensity, but it is preceded by a period of growing brightness and succeeded by one of decreasing light.
In the modern apparatus, however, metallic mirrors are entirely dispensed with, their place being taken by reflecting prisms of glass. The metallic ones had to be continually rubbed to keep them clean, and this soon dulled their brightness, while the glass prisms need only to be wiped carefully, which operation has little effect upon their surface.
It may come as a surprise to some that reflecting prisms are possible. The idea of refraction through a prism is quite familiar. Such forms the essential principle of the spectroscope. Refraction is explained to every school child in order to account for the rainbow. But reflection by a piece of the clearest glass seems a contradiction in terms almost. Yet it is only a question of shape. In some prisms the light is simply bent as it passes through. In others it is bent twice, so that it leaves the prism just as if it had been reflected off a mirror. Both devices are used in the lighthouse. Let us see how they are combined so as to perform the work to be done.
Take first of all the case of a light upon an isolated rock where the warning is needed equally all round. All that is necessary here is to pick up those rays which, if left to themselves, would fall upon the water near the foot of the tower, and those which would waste themselves skywards, and then to gather all the rays into several bundles or beams. We will suppose a simple case in which the light is supposed to give flashes at regular intervals.
We are in the topmost room of the lighthouse, the lantern, as it is called. In the centre there stands the murette or pedestal. In this several columns support a circular platform on the top of which there moves what we might call a turntable, which in turn bears a frame of gun-metal into which are fitted a maze of glass bars triangular in section and curved to form concentric circles. The whole structure, possibly, is of great size. From the floor to the platform is as high as an ordinary man. Indeed around the turntable there is a gallery which forms a roof over our heads, so that it is only after mounting some iron steps on to this gallery that we are able to examine the glass part.