LENS, in optics, an instrument which refracts the luminous rays proceeding from an object in such a manner as to produce an image, real or virtual, of the object (from Lat. lens, lentil, on account of the similarity of the form of a lens to that of a lentil seed). It is generally made of glass, although for special purposes, when it is desired to transmit rays which are strongly absorbed by glass, or for special refraction and dispersion, other substances, such as rock salt or fluorspar, are employed. For the general scientific principles governing the passage of light through lenses and systems of lenses see OPTICS. For the behaviour of lenses in various instruments see MICROSCOPE; PHOTOGRAPHY (for the photographic lens), TELESCOPE. For the lens of the eye see VISION.
A typical method of making a lens involves seven separate processes: (I.) Cutting from a large block of glass a piece of suitable size by means of a rotating thin iron blade, one foot in diameter, whose edge, against which the glass is pressed, is charged with small fragments of diamond in. diameter, and less), the rate of rotation being 24o times a minute. (2.) Roughing, i.e., shaping with carborundum (the grains being of about A. in. diameter) made into a mud with water and smeared on a flat horizontal cast iron plate, of 31 feet in diameter, which rotates sixty times a minute. (3.) Roughing to curve on both sides with similar carborundum on rotating cast iron dishes of the desired curvature. (4.) Trueing to curve on similar, but more accurate, iron dishes with emery mud whose grains are from 0.1 mm. down to 0.05 mm. diameter. (5.) Smoothing the lens— a process like the trueing but in which emery grains down to about 0.01 mm. diameter or less are used. (6.) Polishing with wet jewellers-rouge on polishers consisting of iron dishes whose sur faces are coated with pitch. (7.) Edging the lens on a lathe, by means of a brass plate whose surface is fed with trueing emery, until the lens is circular and of the required diameter; the lens being so chucked that the circular edge is concentric with the thickest (or thinnest) part of the lens. The words in italics are those in general use to describe these processes. The dimen sions, speeds, etc., are subject to modification in different work shops.
History.—The references of Pliny and other ancient writers show that burning glasses were known to them in the form of glass spheres filled with water; and passages from Greek and Roman writers have been cited as showing that they knew of the magnifying properties of lenses, or at least of such glass spheres filled with water. One of the most thorough reviews of the sub ject (T. H. Martin, "Sur des instruments d'optique faussement attribues aux anciens par quelques savants modernes," Bulletino di Bibliografia e di Storia delle Scienze matematiche e Vol. iv. 1871. See also E. Wilde Geschichte der Optik, 1838-43) denies to the ancients all knowledge of spectacle lenses whether for short or long sight, or indeed of any kind of lenses, if we except the spheres of glass filled with water referred to above; and maintains that the lens-shaped glasses or crystals which have been found from time to time among the relics of departed civilizations were made by polishers of jewels for purposes of ornament.
We must come to the end of the thirteenth century for the first authentic mention of the use of lenses, which appears to be that of Meissner (126o-8o) when he expressly states that old people derive advantage from spectacles (see E. Bock, Die Brille u. ihre Geschichte 1903). In the archives of the old Abbey of Saint-Bavon-les-Gand, the statement is found that Nicolas Bullet a priest, in 1282 used spectacles in signing an agreement (see P. Pansier, Histoire des Lunettes 1901). The
first picture in which spectacles are known to have appeared is by Tommaso da Modena, in the Church of San Nicola in Treviso, and is of date 1360. In a sermon delivered February 23, 1305, Giordano da Rivalto stated that "it was only twenty years since the art of making spectacles was discovered" (see P. Pansier, Histoire des Lunettes). It may be accepted, from this and like evidence that the use of spectacles dates from a little prior to 1280; the industry of a host of enquirers has produced no certain evidence that a lens, as such, of any kind had been intentionally made by man previously to this date. William Bourne gives an account, very imperfect, but yet sufficient to show that processes for making lenses were then in use (about 1585) very like those still extant (W. Bourne, A Treatise on the properties and qualities of glasses for optical purposes, Brit. Mus., Lands. ms. 121, un dated but author refers to a book written by him seven years previously which is probably his Treasure for Travellers, 1578). Very different is the account which is given by Baptista Porta of Naples in his famous book Magia Naturalis, Libri Viginti (Frank furt, 1591). The following extract is from bk. xxvii., ch. 21 in the English translation (Natural Magick by John Baptista Porta, a Neapolitane in Twenty Books, 1658), but the matter is identical with the Latin edition of 1591. The translation has, however, rendered the original pilae vitreae—the phrase employed (as by Pliny) to describe hollow glass balls—by "Glass-balls"; and the reader must bear this in mind if he wishes to follow the descrip tion correctly : "In Germany there are made Glass-balls, whose diameter is a foot long, or thereabouts. The Ball is marked with the Emrilstone round and is so cut into many small circles, and they are brought to Venice. Here with a handle of wood are they glewed on, by Colophonia melted. And if you will make Convex Spectacles, you must have a hollow iron dish, that is a portion of a great sphere, as you will have your spectacles more or less Convex; and the dish must be perfectly polished . . . upon the Dish or Ball, there is strewed white-sand, that comes from Vin centia, commonly called Saldame, and with water it is forcibly rubbed between our hands, and that so long until the superficies of that circle shall receive the Form of the Dish, namely a Con vex superficies . . . then rubbing it over again with powder of Tripolis that it may be exactly polished; when it is perfectly polished, you shall make it perspicuous thus. They fasten a woollen-cloth upon wood ; and upon this they sprinkle water of Depart, and powder of Tripolis; and by rubbing it diligently, you shall see it take a perfect glass." Manzini's book (L'Occhiale all' occhio Dioptrica Pratica, Bo logna, 1660) shows that a well developed technique was in use in Italy in 166o; while in 1671 appeared La Dioptrique Oculaire by a Pere Cherubin d'Orleans which not only deals with optics, with telescopes (including binocular) and microscopes, their theory, construction and use, but with the working of lenses, and of vari ous machines invented by himself for that purpose. He writes on all these matters from actual experience, and the tools and ma terials which he used were almost identical with many still widely used. No other writer has given so adequate a description of the making of lenses until quite recently, and his machines are a great advance on those described by Manzini.