Goerz known as the "Dagor," which has since been widely imitated. Almost simultaneously the single "Protars" of C. Zeiss came to be combined in doublet form. These improvements soon showed themselves to be as excellent in practice as they were in theory.
In 1895 Taylor, Taylor and Hobson produced a remarkable and entirely novel anastigmat designed by H. D. Taylor and known as the "Cooke" lens (fig. 3o). It consists of three single lenses, a biconvex crown in front, and a very flat biconvex crown behind, with a biconcave of light flint in between, and close be hind the front crown. Also in 1895 H. L. Aldis calculated for Dallmeyer a new convertible anastigmat consisting of two dis similar compounds, thus giving a choice of three foci. The "Stig matic," as it is called, is still issued but in a non-convertible form. In 1902 H. L. Aldis produced at the new works of Aldis Brothers at Birmingham a much simpler anastigmat composed of a cemented meniscus in front and a single double convex back lens (fig. 31). Several series have been issued and very widely dis tributed. In 1902, again, Ross and E. Busch of Rathenow simultaneously produced anastigmats consisting of four sep lenses, the Ross combination, known as the "Homo being of Jena glasses, while in the Busch "Omnars" as originally constructed normal glasses were employed. In 190.7 H. C. and C. Beck made an other new departure in the Isostigmar, remarkable as violat ing the Petzval condition, the inventor's theory being that the condition does not apply to lenses the individual elements of which are separated by large in tervals (fig. 32). In 1913 J. W.
Hasselkus worked out for Messrs. Ross a singularly fine anastigmat which has since become famous under the name "Xpres." It is a non-convertible doublet, the conspicuous feature of which is a ce mented back combination consisting of three lenses, (I) a bi concave or plano-concave of low refraction, (2) a meniscus of medium refraction, and (3) a biconvex of high refraction.
From 1898 in addition to the above-named, and in partial super session of the doublets of Zeiss, Goerz, Steinheil and Voigt lander, there has been a steady stream of new lenses, mostly triplets—"Tessars," "Pentacs," "Serracs," "Heliars," "Aviars" and others—chiefly remarkable for increase of aperture (fig. 33). A markedly rapid lens is made by Ross to work at f/1.3.
In 1891 T. R. Dallmeyer in London, A. Miethe in Berlin and A. Dubosc in Paris simultane ously worked out telephoto graphic combinations consisting of positive and negative elements, variations in the separation of which produced magnification according to the extension employed. The principles enunciated and clarified by the pioneers referred to above are the active basis of present-day work on lenses.
Current experiments are too widespread to permit of cataloguing here but the 1935 trend should at least be noted: namely, that re cent developments of fine-grained photographic emulsion for cinema film have stimulated a general demand for smaller cameras and lenses of proportionately short focal length, greater consequent depth of field, greater rapidity, and more critical definition.