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Rh been made by Messrs Ross in England. Messrs Steinheil brought out their first “Orthostigmats” in 1893, but, owing to patent difficulties, were unable to manufacture them in Germany, and they were issued later in France and England. They were followed by a second type, which has since been issued in several series by Messrs Steinheil and by Messrs Beck in England (fig. 36). According to Dr R. Steinheil (E. Jb., 1897, p. 22) this lens was an application of two principles recognized by Dr. Steinheil as necessary for the spherical and an astigmatic correction of a lens. He attempted to carry them out in the “Antiplanet,” but was prevented by the want of suitable glass. He found that for an astigmatic correction an objective should have the separating surface between two media concave towards the medium of higher refraction (new achromat), and for

. 36.—Steinheil's “Orthostigmat.”. 37—“Collinear.” Series II.

spherical correction the separating surface should be convex towards the higher refracting medium. A fully corrected cemented lens cannot, therefore, be made with less than three glasses, but with uncemented lenses an air-space may form one of the media. In 1895 Dr D. Kaempfer worked out the “ Collinear" for Messrs Voigtlander, constructed on the same principles as the “Orthostigmat,” type II., and similar to it (fig. 3). It is made in three series. II., f/5·4 and f/6·3; III., f/6·8 and f/7·7 (convertible); IV., f/12·5, and the apochromatic collinear f/8, calculated by Dr H. Harting for three-colour reproduction, &c. (Ph. Jour., 1901, 25, p. 323).

. 38. Series VII. f/12·5. Series VIIa. f/6·3. In 1894 Dr Rudolph extended the application of his principle by combining the old achromat and the new achromat into a single quadruple cemented lens (fig. 38), which, according to T. R. Dallmeyer, was the most perfectly corrected single lens that had been evolved up to 1900, Dr Rudolph having succeeded in obtaining freedom from spherical aberration and astigmatism, and also in eliminating coma (Ph. Jour. 1901, 25, p. 68). These lenses were issued in 1895 as series VII. singly and VIIa., in combinations now known as “Convertible Protars,” and the earlier series VI. and VIa. were withdrawn. The single lenses of series VII., f/12·5, angle 85°, have great an astigmatic flatness of field and only very slight marginal distortion, a condition not realized before in a single lens. The relative rapidities of the double combinations of series VIIa vary from f/6·3 to f/8, according to the lenses used. They are excellent lenses for all general purposes.

In their “Convertible Protars, ” series IV. (1908), f/12·5, angle 60°, Messrs Zeiss have simplified and cheapened the construction of these lenses by the use of new Iena glasses, so that they consist of three instead of four lens elements cemented together, while possessing the same high efficiency as series VII. They are issued as “single” or “double” Protars, f/12·5 and f/6·3 or f/7, also in sets of three or four objectives of different foci, which are combined to give pictures of different angles of view from the same standpoint. With both series when using the “Protar” lens singly, it should be screwed behind the iris diaphragm of the mount, to avoid curvature of the field, and when two such lenses are combined the one with the greater focal length should be placed in front. In 1895 Messrs Goerz patented a double anastigmat, f/5·6, with quintuple single lens components as a convertible lens, for which greater sharpness of definition and intensity, with perfect freedom from astigmatism and distortion in the single lens, were claimed. It was issued in 1898, but, like an earlier analogous quintuplet of Messrs Turner & Reich (1895), it has not come into use on account of the cost and difficulty of construction. The latter firm, however, brought out in 1906 a new symmetrical quintuplet at f/6·8.

A triple anastigmatic combination containing remarkable new features, constructed and patented by H. D. Taylor, was issued in 1895 by Messrs Taylor, Taylor & Hobson under the name of the “Cooke Lens,” and later by Messrs Voigtlander as the "Triple Anastigmat.” It consists of three single lenses, two of them positive crossed lenses of crown glass with high refraction and low dispersion, with their most convex sides outwards, and between them, in front of the diaphragm, a single biconcave of light flint (fig. 39). All these lenses are designed to be free from diaphragm corrections, while the focal power of the negative lens is ma e as closely equal to the combined focal powers of the two positive lenses as may be

. 39.—“Cooke” Portrait Lens. Aperture f/4·5.. 40.—“Cooke” Lens. Series III.

necessary for the flattening of the field and correcting marginal astigmatism. They are not convertible, but arrangements are made for replacing the back lens by a low-power extension lens (Ph. Jour. 1895, 19, p. 64). Series III., f/6·5 (fig. 40), and series IV., f/5·6, are portrait lenses. In the larger objectives of series II. the back lenses are adjustable for uniform shar definition or a soft diffusion of focus. In a later series VI. (1907), f/5·6, this adjustment for diffusion is given to the front lens and is so arranged for portraiture that the diffusing adjustment and iris diaphragm can be operated from the back of the camera while viewing the focusing screen. A special fully corrected “Process” lens on the same general principle has recently been brought out for three-colour work and fine-line reproduction. Another distinctly new type of an astigmatic objective involving several new principles of construction was patented by H. L. Aldis in 1895, and brought out by Messrs Dallmeyer in three series, under the name of “Stigmatic” (Ph. Jour., 1896, 20, p. 117). It also approaches the triplet construction and depends on the introduction of air-spaces between the component lenses. According to Aldis, three conditions must be observed to obtain a flat field iree from marginal astigmatism: (1) The converging lenses must be of high, the diverging of low, refractive index; (2) the converting and diverging components must be separated by a considerable interval; (3) thick meniscus glasses should be used. The first “Stigmatic” was a portrait lens, series I., 1896, f/4. It has been made in two forms, first with a triple front lens, an a back negative system formed of a single thick crown lens of high refractivity

. 41.—Stigmatic Portrait Lens. Series I.. 42.—Stigmatic Lens. Series II.

with a negative cemented meniscus. In the second form (fig. 41) the front component consists of a cemented positive and negative, and both parts of the back component are cemented lenses. All the converging lenses are of dense baryta crown, while both the diverging lenses in the back component are a light silicate crown. It is fully corrected for spherical and chromatic aberration, free from distortion and nearly so for astigmatism. giving equal illumination over a flat field of 60°. Diffusion of focus is obtained by unscrewing the back cell. Series II. (1897) is on the same principle but differs in construction, working at f/6 over an angle of 85° as a universal and convertible lens (fig. 42). The front or back component can be used alone, giving the choice of two focal lengths, 1 and twice the focal length of the complete lens. The principles of its construction were described by T. R. Dallmeyer in Ph. Jour. 1897. 21, p. 167. Series III., f/7·5, will at f/16 give sharp definition over a plate two sizes larger. The single components are not convertible.

In 1897 Messrs Zeiss issued the “Planar,” an objective of large aperture based on the principle of the Gauss telescope objective. It is a symmetrical doublet, each element consisting of three lenses, the two inner ones being a double convex and a double concave, of equal refractive but different dispersive power, cemented together and separated by an air-space from the outer convex meniscus (fig. 43). Its special points are its good colour correction, large relative aperture and intensity, varying from f/3·6 to f/6, with perfectly sharp definition and an astigmatic flatness of field over an angle of view from 62° to 72°. It is a very rapid wide-angle lens useful for instantaneous work with the cinematography and hand cameras, also for portraits and groups, photo-micrograph and enlargements or reductions (see E. Jb., 1898, p. 79, Von Rohr, p. 390, and Lummer, p. 81). Apochromatic planars with reduced secondary spectrum were brought out in 1903 for three-colour photography, and are also useful for astrophotography, the circle of diffusion being very small. The “Unar” (1900), f/4·5 in the smaller and f/6·5 in the larger sizes, angle 65° and 68°, was a further improvement by Dr Rudolph. It 