PHOTOGRAPHIC LENSES Historical.—The earliest form of photographic objective was an adaptation of W. H. Woolaston's single periscopic meniscus lens as applied by him to the camera obscura in 1812. This was achromatised by C. Chevalier for use about 1839 by .Daguerre, but was not corrected for chemical focus. The first photographic lens corrected for chromatic aberration (see later under Construc tion), was a doublet made about 184o by Andrew Ross for H.
Collen, and consisting of two achromatic compounds, one at each end of a tube (fig. 27).' Many single lenses of good per formance were produced by the early photographic opticians, T.
Grubb's "Aplanatic" (1857) and J. H. Dallmeyer's "Wide Angle Landscape" (1865), "Rapid Landscape" (1884), and "Rec tilinear Landscape" (1888) being notable examples.
In 1841 a remarkable ad vance was achieved by Voigt Finder's introduction of a rapid portrait lens designed by Pro fessor Petzval of Vienna (fig. 28), to whom is due the his toric condition that the sum of the focal powers of the individual lenses in a photo graphic combination, multiplied by the reciprocals of their re spective refractive indices, should be equal to zero, or I = o.
,uf This pioneer portrait lens consisted of two dissimilar achromatic combinations widely separated, the front element being a piano 'In the diagrams of lenses which follow, a uniform system of indi cating the nature of the glass employed by means of the shading has been adopted.
convex composed of a biconvex crown cemented to a plano-con cave flint, while the back element was a double convex composed of a biconvex crown separated by an air-space from a concave convex flint. The form was notably improved by J. H. Dall meyer who in 1866 reconstructed the back combination by substi tuting a meniscus crown for the biconvex element, and a shallow concave-convex positive meniscus for the negative meniscus, revers ing the positions, and introducing an air space.
In 1861 J. H. Dallmeyer pro duced a triplet very fully achro matised, non-distorting, and giv ing excellent definition, but since obsolete on account of its bulk and slowness.
Between 1841 and 1866 a number of doublets were brought forward by British and Continental opticians with the object of increasing rapidity and eliminating distortion, and towards the end of this period a simple and efficient type of "rectilinears," "symmetricals" and "aplanats" had become established. In the manufacture of these a high degree of excellence was attained particularly by Dallmeyer and Ross in England and by Steinheil in Germany (fig. 29). Wide-angle symmetrical doublets with greatly deepened curves appeared about the same time, some exhibiting remarkable qualities of definition combined with covering power.
Dallmeyer's "Rapid Rectilinear," f/8 (1867) set what remained as the standard for photographic lenses for ordinary purposes until the perfection of the anastigmat. Among slower combinations the "Portable Symmetrical" of Ross was remarkable for its flatness of field and a compactness which enabled a number of lenses of widely different foci to be fitted into the same flange.
The issue in 1886 by Schott and Genossen of the "Jena glasses" formulated by Professor Abbe placed in the hands of photographic opticians a means of eliminating astigmatism by setting high re fractivity with low dispersion against low refractivity with high dispersion. Some of the early Jena glasses were unstable and prog ress in construction was at first slow. In 1888 Dr. Shroeder worked out for Ross an anastigmat afterwards known as the "Concentric Lens" but the first thoroughly successful objec tive made with Jena glasses was that designed in 1889 by Dr. Paul Rudolph of the Zeiss firm, and classed as Series II. f/6.3. The details of construction are to be found in the Eder Jahrbiicher for 1891 and 1893.
This excellent lens was followed by several other anastigmatic singles and doublets, a new line being struck out by the symmetri cal cemented doublet of C. P.