Hyperradial,1,330millimetres(52·3 inches)focal distance.
1st order,920”(36·2 ”)”
2nd ”700”(27·6 ”)”
3rd ”500”(19·7 ”)”
3½ ”375”(14·7 ”)”
4th ”250”( 9·8 ”)”
5th ”187·5”( 7·4 ”)”
6th ”150”( 5·9 ”)”

The most powerful apparatus used to-day, however, is that known as the “hyperradiant,” and it is the largest which has yet been devised. For this, lighthouse engineering is indebted to Messrs. Stevenson, the engineers to the Commissioners of Northern Lighthouses. It was first suggested as far back as 1869, and experiments were carried out which emphasized the fact that such an apparatus was required, since it was found that when large gas-burners were used much of the light in revolving apparatuses was out of focus and escaped condensation. The Scottish engineers thereupon suggested that an apparatus should be used having a focal distance of 1,330 millimetres, or 52·3 inches. In fact, they went farther and suggested even larger apparatuses, but this idea has not matured. But it was not until 1885 that Messrs. Stevenson had such a system manufactured, and then it was tested at the South Foreland beside the powerful lenses which had just been built for the new Eddystone and the Mew Island lighthouses. The merits of the theories advanced by Messrs. Stevenson were then completely proved, for it was found that with a ten-ring gas-burner the hyperradiant apparatus threw a light nearly twice as powerful as that given by the rival lenses with the same burner.

Fig. 10.—Quadruple Flashing Apparatus: Four Panels.

(By permission of Messrs. Chance Bros. and Co., Ltd.)

At the present moment the hyperradiant is regarded as the ultima thule of lighthouse optical engineering, and Messrs. Chance Brothers and Co., of Birmingham, have built some very magnificent apparatuses of this order. At present there are not more than a dozen such powerful lights in operation. Three are on the English coast, at Bishop Rock, Spurn Point, and Round Island, respectively; two in Scotland, at Fair Isle and Sule Skerry; two in Ireland, at Bull Rock and Tory Island; one in France, at Cap d’Antifer; one in China, at Pei Yu-shan; one in India, at Manora Point, Karachi; and the Cape Race light in Newfoundland. The hyperradiant apparatus is a massive cage of glass, standing some 12 feet in height, and, as may be supposed, is extremely expensive.

There is another point in lighthouse optics which demands explanation. This is the term “divergence,” which plays an important part in the duration of the flash. In speaking about focus, the engineer follows somewhat in Euclid’s footsteps in regard to the definition of a point; in a way it is equally imaginary. The focal point does not mean the whole of the flame, but the centre of the luminous source, and, as is obvious, it is impossible to secure a flame without dimensions. It may be an attenuated, round, oval, or fan-shaped light—the result is the same. The focal point is the theoretical centre of the luminous source, and the rays, coming from the top, sides, and bottom of the flame cannot come from the true focus. If they did, all the light from one panel would be emitted in absolutely parallel lines, and therefore in a revolving apparatus the beam would pass any given point on the horizon in an infinitely short period of time—to be precise, instantaneously. But the ex-focal rays of the flame, in passing through the lens, emerge at an angle to those coming from the absolute centre, so that the whole beam becomes “diverged,” and throws a cone of light from the lens. Consequently the beam occupies an appreciable period of time in passing a given point on the horizon.

As may be supposed, the intricate character of the lenses constituting the optical apparatus of the modern lighthouse demands the highest skill and infinite care in their preparation, while the composition of the glass itself is a closely guarded secret. There are less than half a dozen firms in the world engaged in this delicate and highly specialized work, of which France claims three, Germany one, and Great Britain one. All the lighthouse authorities of the various nations have to secure their requirements from one or other of these organizations. The industry commenced in France, and for many years the French reigned supreme. Then it contrived to make its entrance into England, and was taken up by the family of Chance in Birmingham, who soon proved themselves equal to their French leaders.

Fig. 11.—Red and White Flashing Apparatus.