The rating or classification of the lights varies according to the inside radius or focal distance of the lens--in other words, the distance from the centre of the light to the inner surface of the lens. The main groups are as follows: Hyperradial, 1,330 millimetres (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. [Illustration: 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.