Magnifying Lens (Magnifier).—A lens or a series of two or three lenses used to increase the apparent size of an object, mounted so that one, two or three may be used separately or superposed one upon the others, and in con venient form for the pocket.
Pebble Lens (trade term).—A spectacle lens made of rock crystal which is harder than glass and has a different angle of refraction.
Photographic Lens (also photographic ob jective).—A lens or a combination of lenses designed for photographic purposes. It is made in a great variety of types, the simplest being the single achromatic convex meniscus lens. The form in most common use is composed of two separated achromatic menisci, with their concave surfaces toward one another, known commercially as symmetrical, rectilinear or aplanat. The portrait lens, another type, gen erally of large diameter, having great light gathering power, is composed of two separated achromatic lenses, one cemented and the other uncemented (Petzval type). The most modern photographic lens is the anastigmat, invented by P. Rudolph, which is free from astigmatism, a fault present in all earlier types. The new varieties of optical glass made in Jena were first successfully employed in these lenses, by means of which greater perfection in other di rections was also attained.
Punktai spectacle lens corrected for astigmatism over a field of 80 degrees. See SPECTACLES.
Spectacle lens used to correct vision and when two are combined by a bridge which rests on the nose and provided with bows which clasp the temples is now generally termed spectacles.
The early history of lenses is quite vague. While the magnifying property of glass globes filled with water and presum ably of glass beads was known, there is no authentic information that lenses were made and used. There is in the British Museum a piece of rock-crystal about the size of a modem spectacle lens, cut to a piano convex form which was found by Layard during the excavations at Nimroud. Instead of having a spherical sur face, it is made up of a series of facets and the crystal is permeated by cloudy stria.
Alhazen, who died about 1052, first described the magnifying effect of simple lenses. Spec tacle lenses were well known in the 13th cen tury, and their invention is credited to Salvino d'Armato degli Armati, about 1255. The com bination of two single lenses, thus forming a compound microscope, for the purpose of magnifying objects, is believed to be due to Hans and Zacharias Janssen of Middleburg, Holland, about 1590. The discovery of the telescope, which is credited to Galileo, about the year 1610, was in its original form a com bination of convex lens of long focus with a concave lens of short focus, and this form is still retained in the ordinary opera glass, which is designated as a Galilean telescope. The achromatic lens was the next important inven tion and was made by Dolland in England in 1758. The making of lenses was revolutionized in 1886 by the announcement of 19 new kinds of glass by the Jena Glass Works ofJena., Germany. In 1888, 24 other kinds were placed on the market; and eight more in 1892. Not all of these were suitable for making lenses, but some of them proved superior and new com binations with former glasses were possible, resulting in some of the most remarkable in strumental lenses theretofore produced. In July 1914 a practical glass maker was added to the force of the Bureau of Standards of the United States, and this bureau has since made some of the finest optical glass ever produced.
See GLASS; JENA GLASS; LIGHTHOUSE; MICRO SCOPE ; OPTICS; PHOTOGRAPHY; STEREOPTICON; TELESCOPE. Consult Bolas, T., and Brown, G. E., 'The Lens) (New York 1902); Fraprie, F. R., 'How to Choose and Use a Lens' (Boston 1915) ; Hovestadt, H., 'Jena Glass, and Its Scien tific and Industrial Applications) (London 1902) ; Prentice, C. F., 'Ophthalmic Lenses and (Chicago 1917); Thorington, J., 'Re fraction and How To Refract) (Philadelphia 1914) ; United States Bureau of Standards Re print 122, 'The Resolving Power of Objectives) (Washington 1909); Wood, R W., 'Physical Optics) (New York 1911).