gether in the components was subsequently increased to four, and even five.
The single components of such a lens, when used behind the diaphragm, form excellent single anastigmats, perfectly corrected for chromatic and spherical aberrations, but not for coma, and showing, at least at the edges of the field, a little distortion and curvature, but very suitable for landscape work. The focal length is usually 104. Convertible Anastigmats of Two Com binations of Cemented Glasses. The excellent results obtained by Rudolph incited a great number of opticians to use the new combina tions of glasses, but they sought for a solution of the problem in another direction—returning to the convertible symmetrical lens. The Ger man factories being almost the only ones at this period to possess research departments, the first about 1-8 times that of the complete lens, and the camera extension is rather more than doubled. The maximum effective aperture is then equal to the actual diameter of the largest stop, and the relative aperture about 40 per cent (mean value) less than that of the complete lens.
Meanwhile (1895) Rudolph, after having tried and rejected this arrangement, worked out a single anastigmat of four cemented glasses.
(1893) of the new category to appear was the double-anastigmat calculated by E. von Hoegh (Fig. 77), covering a field of 72° at F/8 (after wards increased to F/7.7 and then to F/68), and almost 90° at F/22, forming an excellent universal lens. A number of variants followed, from process lenses covering about at Fix' to the French eurygraphe lens, in which the aperture was enlarged to F/5 with the same field of 90°. The number of glasses cemented to which could be used either separately (field about 5o° at a maximum aperture of F/12-5) or in pairs, forming either a symmetrical lens at F/63 or an unsymmetrical lens (Fig. 78), at an aperture from F/7 to F/7-7 (according to the degree of dissymmetry) covering about 45° at full aperture and at F/25. This arrangement has since been adopted by the greater number of opticians, some of whom have pushed the aper ture to T1,5 and that of the single components to F/9. These convertible unsymmetrical anastig
mats form the simplest " sets of lenses " or " casket lenses," giving the photographer a range of three focal lengths in steps, generally in the ratio of I : 1-6 : 2 (or i : 1-5 : 2-2), the focal length of the complete lens being taken as 1. Care should be taken always to place the single lens, when used by itself, behind the diaphragm, and, when re-assembling the com plete lens, to place the shorter focus component behind, the longer component being placed in front.
105. Unconvertible Anastigmats consisting of Uncemented Lenses. A completely different conception of the anastigmat led H. D. Taylor (1893) to design numerous types of specialized struction of symmetrical convertible anastig mats, many opticians have sought to take advantage of the freedom given by combinations of separated lenses. One of the first of this type appears to be that calculated by von Hoegh (1898). Originally the maximum aperture was only about F/Io for the complete system, but it was extended by Zschokke successively to F/7-6 (1903), and then to F/5.5 and F/4.5, according to the focal length (Fig. 81), the field being about 6o° at the maximum aperture and 65° at a very small aperture.
The presence of eight glass-air reflecting surfaces has some drawbacks (§§ 56 and 57). The maker of the above type therefore recom mends that the lens should preferably be used lenses, all consisting of a system of three separ ated lenses, among which the model shown in Fig. 79 is particularly designed for hand cameras, covering 70° at a maximum aperture of F/6-5. Among other forms by the same designer may be noted a portrait lens at F covering about 40°, a wide-angle covering 97° at F/6-5, and a process lens at F/8 (F/Ii and F/16 for the very great focal lengths) covering 55° at full aperture and about 8o° at F/32.