22 SCIOPTICON MANUAL. 



Among the modified forms, the Dallmeyer Landscape 

 Lens, which consists of three lenses" cemented together, 

 a central one of flint-glass and two outer ones of different 

 kinds of crown-glass, gives better results. The stop B B 

 is generally one-fifth of the focal length distant from the 

 lens, and consequently cuts off much of the light. In 

 the earlier days of photography a person had to sit in 

 front of such a lens, in a strong light, for several minutes. 

 That in this way no artistically perfect pictures could 

 be made is self-evident, and so it became necessary for 

 portrait photographers to have a lens that would work 

 satisfactorily with a larger opening. 



THE PORTRAIT OBJECTIVE. 



This invention is no accident, but the result of a 

 thorough theoretical calculation. It is a double ob- 

 jective with two unequal lenses, with or without central 

 stops between. 



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Fiff. 13. 



The front lens A consists of a biconvex crown, and 

 an almost plano-concave flint-glass lens, cemented to- 

 gether with Canada balsam. The back lens B consists 



