A few years ago another magnificent light was brought into service in the North Sea by the installation of electricity in the lighthouse of Heligoland. With characteristic Teuton thoroughness, the Germans discussed the question of the illuminant for this beacon in all its bearings, and resolved to introduce the most powerful light possible. This decision was influenced by the dangerous character of the waters washing the island, as it is flanked on all sides by highly perilous ridges and sandbanks, which must become accentuated owing to the heavy sea-erosion that prevails. The German authorities investigated the various electrical installations that had been laid down for lighthouse work, with a view to discovering the most suitable system, the advantages and defects of existing electric lights, and how the drawbacks might be overcome most successfully. Meantime the famous Siemens firm discovered a means of grinding glass mirrors into parabolic form, and this discovery was accepted as the solution to the problem. In this type of mirror the back is silvered. The metallic polished surface is protected completely from mechanical injury and from all possibility of tarnishing. The inventors claim that mirrors so prepared are able to compete successfully with lenses and totally reflecting prisms--in fact, it was maintained that the silvered glass parabolic mirror possessed the advantages of greater reflecting power and enhanced accuracy, with less divergence of the beam of light. Owing to the perfection of the lenses and prisms system of lighthouse optics, the introduction of arc lights in conjunction with parabolic mirrors was received with considerable hesitation. In order to dispel these doubts, the above-mentioned firm forthwith embarked upon an elaborate series of comparative tests at Nuremberg to ascertain the relative value of the two systems, and as a result of these experiments they concluded that quite as good an effect is obtainable with the arc and parabolic mirror as with the best examples of any other method. Accordingly, the authorities decided to install the system in the Heligoland lighthouse. They stipulated that the intensity of the beam of light should be at least 30,000,000 candle-power, with a maximum current of 100 ampères. The duration of the flash was to be one-tenth of a second, followed by eclipses of five seconds’ duration. The electrical engineering firm entrusted with the contract fulfilled these conditions by mounting three searchlights spaced 120 degrees apart upon a rotating platform. That is to say, each light is projected outwards from a point equal to a third of the circumference of a circle. The mirror diameter was settled at 75 centimetres (29½ inches) and the focal length at 250 millimetres (10 inches), the current being taken at 34 ampères when the table made four revolutions per minute.