The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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crown glass. This has been modified |by Dallmeyer, by Grubb, and the noted American optician Morrison ; but all are con-
Fig. 78.
structed on the principle of the same lens (fig. 77). Professor Petzval calculated at the same time a landscape lens (fig. 78), which was not introduced commercially till 1857 ; and an English
Fig. 80.
optician, in 1858, having suggested a lens with a concave glass in place of the diaphragm to lengthen the focus and flatten the field, Dallmeyer introduced his famous triplet (fig. 79),