The examination of the objectives can only be attempted when the different faults of the objective are known.
Photog., 1891, 5, p. 225; 18 93, 7, p. 221), cemented objectives of thin lenses permit the elimination of spherical aberration on the axis, if, as above, the collective lens has a smaller refractive index; on the other hand, they permit the elimination of astigmatism and curvature of the field, if the collective lens has a greater refractive index (this follows from the Petzval equation; see L.
This artifice is specially adopted in objectives for astronomical photography ("pure actinic achromatism").
On this account the lines D and G' are united for ordinary photographic objectives; the optical as well as the actinic image is chromatically inferior, but both lie in the same place; and consequently the best correction lies in F (this is known as the " actinic correction " or " freedom from chemical focus ").
Abbe succeeded in computing microscope objectives free from error of the axis point and satisfying the sine condition for several colours, which therefore, according to his definition, were " aplanatic for several colours "; such systems he termed " apochromatic."