Of the many industries developed by the war, the production of optical instruments offers a striking example of rapid progress. Before 1914, the optical glass industry of Europe was largely in the hands of Germany and Austria, and the outbreak of hostilities meant the total closing of that market to the Allies. The lack of optical instruments thus occasioned was at first a source of grave national peril, since optical glass provides, as it were, eyes for both Navy and Army. The eyes of the guns are the range-finder, the director, the sighting telescope, periscope, prism binoculars, and other instruments for observing fire and correcting the aim; the tank would be blind without its periscope, and observations are made from aircraft by means of photographic cameras and lenses. At sea, the tale is repeated; the submarine requires at least one eye, and the submarine chaser needs many, while, by means of optical instruments, the naval gunner can fire at a target which is about 15 to 20 miles away. The very health of the army depends, in great measure, on optical glass, since the Royal Army Medical Corps fights malaria and other diseases due to parasites, which must be magnified by a microscope a thousand times before they can be identified. Hence, the solution of the problem of optical munitions was a vital matter in the early days of the war. With characteristic energy, Great Britain set to work and soon restored a languishing trade. The task was enormous; the industry had to be revived from its very foundations. The production of the peculiar types of glass required for optical instruments in itself presented a formidable obstacle, even its principal ingredient, a special quality of sand, being formerly derived mainly from Fontainebleau and Belgium. But by widespread investigation efficient substitutes were soon discovered, the problem of mixing the ingredients was at length solved, formulæ for special glasses devised, and we are now producing large quantities of optical glass of perfect quality. The production of the raw material was, however, only a first step in obtaining an adequate supply of optical instruments. Numbers of delicate processes stand between the rough glass and the finished implement. The glass must be cut, ground, and curved exactly to the requisite design, which in itself takes many days of high mathematical computation; it must be smoothed and polished, cleaned with meticulous care, and adjusted to a nicety in the particular instrument for which it is fashioned. The difficulties and pitfalls are incalculable; from start to finish the glass obeys no fixed laws, but answers only to the skilled handling of the scientist and craftsman. 'Optical glass is the mule of materials', comments a recent writer with sincerity.