Fraunhofer prepared glasses which reduced the secondary spectrum; but permanent success was only assured on the introduction of the Jena glasses by E . Abbe and O . Schott.
2.
These chromatic errors of systems, which are achromatic for two colors, are called the " secondary spectrum, " and depend upon the aperture and focal length in the same manner as the primary chromatic errors do.
3.
It increases rapidly with the aperture, and is more important with medium apertures than the secondary spectrum of par-axial rays; consequently, spherical aberration must be eliminated for two colors, and if this be impossible, then it must be eliminated for those particular wavelengths which are most effectual for the instrument in question ( a graphical representation of this error is given in M . von Rohr, " Theorie und Geschichte des photographischen Objectivs " ).
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