If a lens is undercorrected, then some prisms will shift the colour error slightly to bring red more into focus while at the same time making blue more out of focus.
Quote:
What I have found in some older Zeiss documents mention that to get maximum performance from their
refractors they suggest a 90deg prism diagonal.
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In the days of film, some refractors were made to be better corrected at the wavelengths that film was most sensitive to, which was blue. Perhaps the reference to using a prism in the Zeiss documents was to balance out this undercorrection.