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HANDBOOK OF PHOTOGRAPHY 



Another modification of the Cooke lens is the Aldis lens (Fig. 31) designed in 

 1901 by H. L. Aldis, to cover 35° at//6. In this type the front two lenses are thick- 

 ened and cemented together to form a low-power negative system of such a shape as to 

 correct the aberrations of the single positive lens forming the rear element. 



Fig. 29. — Taylor-Hobson 

 //2.5 Speedic lens. 



Fig. 



30.— Zeiss //I. 

 Sonnar lens. 



Fig. 31.— Aldis 

 lens; modification of 

 Cooke lens. 



The Four-piece Lens. — In connection with the Cooke lens above, it was mentioned 

 that the Petzval sum can be reduced by separating the positive and negative elements 

 of an achromatic doublet. If two such separated doublets are mounted symmetrically 

 about a central stop, a lens is obtained which offers even more possibilities for a good 

 design than does the Cooke lens. Two independent series of designs based on this 

 general principle have been developed, one in which the four lenses are all biconvex or 

 biconcave and the other in which all four lenses are meniscus-shaped. The first form 

 is exemplified by the Goerz Celor //4.5, designed by von Hoegh in 1898 (Fig. 32). 

 Later modifications of this type are the Goerz Dogmar, the Steinheil Unofocal, and 

 the Taylor-Hobson Aviar. The second form may be said to have originated in the 

 Alvan Clark lens of 1889 (U. S. Pat. 399499) in which two Gauss-type telescope 

 objectives were combined together with their concave sides facing a central stop. 



Fig. 32.— 

 Goerz Celor 



//4.5. 



Fig. 33. — Ross Homo- 

 centric. 



Fig. 33a. — Bausch 

 and Lomb Metrogon. 



Fig. 34.— Unar 

 of Rudolph. 



lens 



The Gauss telescope objective consists of a meniscus-shaped crown and fhnt elements 

 in close contact and is characterized by being spherically corrected at two different 

 wavelengths, thus having exceptionally good spherical correction throughout the 

 who'e spectrum. One of the first anastigmats to embody this principle was the 

 Ross Homocentric (Fig. 33) and the identical Meyer Aristostigmat designed by 

 Kollmorgen in 1902. The type is still being adopted for some purposes such as the 

 //lO process lens of Bausch and Lomb. An extreme example of this form is the 

 Richter lens (U.S. Pat. 2031792), the same type of construction being adopted in the 

 Bausch and Lomb Metrogon (Fig. 33a). The glass types employed in these sym- 

 metrical four-piece lenses are generally barium crown and light flint, but it is quite 

 possible to satisfy the Petzval sum with old glasses as was shown by Martin in the 

 Busch Omnar in 1902. Rudolph tried mixing the Celor and Homocentric types by 

 using the front half of the Celor with the rear of the Homocentric types in his Unar 

 lens (Fig. 34), but it was not long manufactured. The Wray Lustrar is also of this 

 general type (Fig. 35). 



Variations of the Celor type soon appeared. For example, Goerz made the two 

 negatives into cemented triplets in the Alethar process lens (Fig. 36), and Rudolph 



