Mirrors for searchlight purposes are of very varied forms and sizes, the shape depending largely upon the particular form of beam which they are designed to project. For many purposes a parabolic form is required, while in others, where a flat, fan-shaped beam is to be produced, a form having an elliptical section in a horizontal plane and a parabolic section in the vertical plane is required. In most cases these mirrors are produced by bending plates of glass, previously raised to the necessary degree of heat, over suitably shaped moulds, the surface being subsequently re-polished to remove any roughness resulting from the bending process. Another type of mirrors is that known as “Mangin,” which has two spherical surfaces placed eccentrically in such a way that the centre of the mirror is considerably thinner than the periphery; in this type of mirror the reflecting action of the back surface is modified by the refracting action of the front surface, but both are spherical, and can therefore be accurately ground and polished by the usual mechanical means. Such mirrors are manufactured of single pieces of glass up to 6 feet in diameter.
APPENDIX
BIBLIOGRAPHY.
The existing literature of glass manufacture is so limited that a complete bibliography could almost be given on a single page; in the English language, in particular, there are exceedingly few books and papers on the subject. The French and German literature of the subject is a little more extensive. In giving a list of the works, and more particularly in referring to those which he has consulted in the preparation of the present volume, the author thinks it will be an advantage to indicate their scope, and, to some extent, what he believes to be their value, in order to save the student the trouble of seeking out comparatively inaccessible works only to find that they contain little that is of value for his purpose.
English Books and Papers on Glass Manufacture.
The Principles of Glass Making (George Bell & Sons). By Powell & Chance. An elementary book giving a clear and concise account of the older processes, more especially in connection with flint and plate-glass.
Glass. Articles in 9th Edition of Encyclopædia Britannica. A detailed account of processes, more or less covering the entire subject, but the processes described are mostly obsolete at the present time.
Glass. Article in Supplement to 9th Edition of Encyclopædia Britannica. By Harry J. Powell. A brief summary of more recent developments. Particularly valuable in reference to artistic English flint-glass.
Jena Glass. By Hovestadt, translated by J. D. and A. Everett. Contains a full account of the scientific work on glass and its practical application, done in connection with the Jena Works of Schott. Particularly interesting in connection with the subjects of Chapters I., II., XII., and XIII. As the title indicates, the book is written from the Jena point of view, and scarcely does justice to work done elsewhere. The book has gained considerably at the hands of the translators.