THE DEVELOPMENT OF THE PHOTOGRAPHIC OBJECTIVE 



41 



pensate the inevitable spherical undercorrection of the new achromat rear element. 

 Similarly, the glasses for the rear element were so chosen that the Petzval sum of the 

 rear would compensate that of the front element. The shapes of the lenses were also 

 chosen so as to correct the coma and astigmatism of the whole system. 



The Protar lens, as first made, operated at //7.7 or less. However, its aperture 

 was subsequently raised to //4.5 by changing the rear component into a triplet (Fig. 



Fig. 12. — Goerz 

 Dagor. 



Fig. 13. — Symmetrical 

 double-protar lens. 



Fig. 14. 



-Voigtlander Col- 

 linear. 



11) in which the positive barium crown element was divided into two parts placed 

 one on each side of the light flint negative element. 



In 1893, as a result of the simultaneous and independent activities of Rudolph at 

 Zeiss and von Hoegh at Goerz, the old and new achromat elements of the Protar were 

 combined into one. The Zeiss Triple-Protar and the Goerz Dagor were practically 

 identical designs, consisting of a symmetrical arrangement of two cemented triple 

 elements (Fig. 12). Each element was in external form a thick meniscus lens, this 



Fig. 15. — Watson Holo- 

 stigmat. 



Fig 



-Reichert Com- 

 binar. 



17. — Rietzschel 

 Linear. 



shape helping to reduce the Petzval sum in addition to flattening the field. The 

 refractive indices were in the order high-medium-low while the F-numbers of the 

 glasses were, respectively, high-low-high. Thus, if an imaginary line is drawn to 

 bisect the middle lens of each element, the outside doublets are evidently new achro- 

 mats and the inside doublets are old achromats. 



In the same year, Rudolph designed the convertible double-protar lens (Fig. 13) 

 in which the new and old achromats were directly cemented together forming quad- 



FiG. 18. — Goerz Pantar. Fig. 



19. — Goerz Convertible 

 Anastigmat. 



Fig. 20.— Half of Turner- 

 Reich convertible lens. 



ruple elements, which were then mounted symmetrically in pairs about a central stop. 

 Because of the high degree of correction of these elements, they could be used sepa- 

 rately as anastigmatic landscape lenses at //1 2 or combined together in pairs to make 

 a "set" of interchangeable lenses. Thus by having a "set" of three lenses of different 

 foci, six possible focal lengths could be obtained. This design is still largely used by 

 commercial photographers. Other variations of these symmetrical cemented anastig- 

 mats soon followed. Some typical examples are given in Table III: 



