The Dictionary Of Photography

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

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the object, as in fig. 75. Then Wollaston's meniscus (fig. 76) came to be recognised as a means of extending the definition ; and in 1840, Chevalier, a Paris optician, still further improved it by a different method of achromatising the lens. But in the following
Fig. 74-
Fig. 75.
year, through the agency of Voigtlander, a practical optician, a lens designed by Professor Petzval, a mathematician of Vienna, was made and introduced commercially in 1841. This was the portrait lens; and it is a remarkable fact that it is the model for
Fig. 76.
Fig. 77-
the finest portrait lenses of the present day. The above (fig. 77) is a sketch of Petzval's original portrait lens :- - The dark shaded parts are crown, the light shaded parts are flint glass, and, as will be seen, the front combination exists of a double-convex crown cemented to a double-concave flint, and the back of a flint concavo-convex separated from a double-convex lens of