Page:The Development of Navies During the Last Half-Century.djvu/249

 object, by which the guns may be directed with accuracy. Among the many uses to which electricity is now applied on board ships of war not the least important is its conversion into a strong beam of light which, when directed from the vessel, illuminates objects within a certain distance. This is universally termed an electric search light. The apparatus consists of a dynamo-electric machine for producing the electricity, a conducting cable for conveying the electricity to the lamp, where it is converted into light by means of two carbon rods, and the rays then concentrated into an intense beam by a reflector.

On a small scale this has been done before, and I believe a search light produced by a voltaic battery and a parabolic reflector was used in the Crimean War. This was little more than a toy, but the dynamo-electric machine, since introduced and perfected, has enabled great advances to be made, while the spherical reflecting mirror, devised by Colonel Mangin, is no less important in utilising the light thus produced. This reflector is of glass, ground so that the circumference is thicker than the central portion. This mirror collects and concentrates all the rays which impinge on it from the carbons in front, and then projects them forward in an intense beam of light. In clear weather small objects can be clearly discerned at upwards of a mile, when the ray is thrown on them, but in fog or mist the light has no penetrative power. There appears to be a deficiency of red rays, which are not absorbed by aqueous vapour to the same extent as the rays of other colour in white light. As