Page:The New International Encyclopædia 1st ed. v. 13.djvu/655

MIRROR. 583 MIRROR OF KNIGHTHOOD. 1300; nnd judginf; from tiiose still in existence, they were very riulc coiitrivunce.s, compared with those of modern make. It was not until l(i73 that the making of mir- rors was introduced into England, and the in- dustry has since developed in Europe and America to a point where it is a very important niamifactnre: and mirrors can be jjroduced of any size to which plate glass can be east. For many centuries mirrors were made accord- ing to the process originally introduced at Venice, by backing a sheet of glass with an amalgam of mercury and tin. The surface was overlaid with sheets of tinfoil, rubbed do«Ti smooth, and the whole covered with quicksilver, which immediately formed an amalgam with the tin. The su])eriIuous mercury was then run oil' and a woolen cloth held firmly over the surface, by means of iron weights. After this pressure had been continued twenty-four hours, more or less, the weights and cloth were re- moved and the glass ])laeed on a table with a movable toj). which was gradually inclined until the unanialgamated quicksilver had entirely drained away, and only the surface of perfect amalgam remained adhering to the glass. This process, which was long used, was open to many objections, not least of which was its extreme unhealthfulness for the workmen. The process was also long and tedious, and at best made an imsatisfaetory mirror, reflecting less than half t!ie rays of light. The first attempt to back the glass with silver was made by Liebig in 1830. and diflferent solu- tions were proposed by other chemists, all of which produced mirrors that were satisfactory for a short time, but finallj' became spotted. In 185.5 Pcttijean jiatented a process which, with various modifications, is the one now in general tise. The method of mirror manufacture com- mon in America may be described as follows: The raw stock or plain plate glass reaches the factoiy carefully packed in eases of immense size. The glass is first thoroughly inspected and all defects marked. It then goes to the cutters' department, where it is cut into the proper sizes. Thence it is moved to the beveling de- partment, where it is beveled and polished on rapidly revolving emerj' wlieels of varying de- grees of smoothness, the plates^some of them of enormous size — being lightly held against the wheel by the workmen. After both surfaces, incluiling the beveled edge, have been reduced, as nearly as possible, to a condition of perfect smoothness, the glass is passed on to the silver- ing di'partnient. Here it must be thoroughly cleansed, and so delicate is this operation that n sppiially distilled water is often required I r the purpose. The glass is now ready for the I -^rntial process of silvering. The nitrate of -ilvcr is dissolved in ammonia and is precipitated by a solutfrm of tartaric acid. The glass is placed on warm tables and the solution poured over it. The heat lielps the silver to i)recipitate and adliere to the glass. The silver back re- ceives a coat of shellac and then of paint, which completes the process. Silvered mirrors reflect from 20 to 2.5 per cent, more of light than those backed with quicksilver. The optical considerations involved in reflec- tion will be foimd fully discussed together in the article on Limit, but lirief mention may here be made of mirrors whose reflecting surfaces are other than i)lane. In a concave spherical mir- ror we have distant rays of light or heat brought to a focus and a real image form. Conversely, if a point source of light is placed at the focus of the mirror, a parallel beam of light results. The first idea is made use of in the rellecting telescope (see Telescope) , while the latter is employed in the search liglit (q.v.). A pariiholic minor is one "in which every section through the principal axis cuts the sur- face in a i>arabola, so that rays from a light placed at the focus are all reflected parallel to the axis and, conversely, parallel rays are brought to the focus." The reflector of a loco- motive headlight is thus constructed. See AbERKATIOX, SPIIERICAi; LiGHT. Cylindrical mirrors do not play as important a part in optics and optical instruments as those of S])herical and ])arabolic cross-section, but their etl'ects are sometimes interesting. By using a glass that is curved instead of flat, the reflected shape of the object will become dis- torted: a concave cylindrical mirror lengthening it at the expense of width, and a convex mirror producing the opposite effect. I As heat is reflected as well as light from the surface of a mirror, a concave mirror may be used to bring rays of light to a focus. In this way combustible substances may be set on fire at a distance from the reflector whence they receive their heat. The Arcltimedean mirror was made on this plan. A series of mirrors set in a concave curve concentrated the rays of light iipon an enemy's ship, causing it, according to the story, to bum. MIRROR CARP. An artificial variety of carp (q.v. I with very large scales in two or three rows along the sides of its body, which is other- wise bare. MIRROR FOR MAGISTRATES, A. A long series of poems on incidents in English history showing the tragedies in the lives of great men. The plan was suggested by Boccaccio's Falls of Uluntrious Men and Lydgat^-'s Falls of Princes. and was devised by William Baldwin, George Ferrers, and Saekville. who wrote a general introduction called the Induction. It was partly printed in 15.55. when it was stopped by Lord Chancellor Gardiner. It was licensed, however. in 1559, and then contained nineteen metrical biographies, lieginning with Tressillian in Rich- ard XL's reign. New editions, witti additional lives by various writers, appeared in 15(>3. 1574, 1578, 1587, and 1010. and a reprint of the whole by Haslcwood in 1815. Some of the best-known contributors are Thomas Saekville, who.se JniJuc- lion and Comphiint of f<lafforil. Dnke of Jinck- hif/ham. are the most valuable parts of the work; Michael Drayton, Thomas Churchyard, and John Skelton. MIRROR OF KNIGHTHOOD, TnE. A translation of a Spani'sh romance. Cnviillcro del Fi'ho. the Knight of the Sun. which tells the adventures of Febo and his brother Rosiclair. It belongs in a sense to the Amadis cycle of romance, as the father of the Knight of the Sun was related to Amadis. The Spanish version was evidently the work of several, and was left un- finished." The translation of the romance into English was printed in 1578. See Dunlap's History of Prose Fiction.