But in the following 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 the finest portrait lenses of the present day. The following (fig. 7) 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 crown glass, the flint concavo-convex having such a negative refractive power as to completely balance the
positive aberration of the whole combination. This has been modified by Dallmeyer, by Grubb, and the noted American optician Morrison; but all are constructed on the principle of above lens (fig. 7). Professor Petzval calculated at the same time a landscape lens (fig. 8), which was not introduced commercially till 1857 and an English optician, in 1858, introduced a lens having a concave glass in place of the diaphragm to lengthen the focus and flatten the field, and Dallmeyer introduced his famous triplet (fig. 9), which at the present time is much used and admired.