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    Re: Sextants with Polarizing filters
    From: Bill B
    Date: 2006 Jan 26, 23:09 -0500

    > Actually "ortho" means vertical thus 90 degrees to the horizontal plane
    
    Wow, Through the (polarized) Looking Glass
    
    http://photography.about.com/library/glossary/bldef_ortho.htm
    
    "Definition:
    
    Orthochromatic films were first produced by Hermann Vogel in 1873 by adding
    small amounts of certain aniline based dyes to ordinary photo emulsions,
    work that was extended by others including J M Eder, who introduced the use
    of the red dye erythrosine in 1884.
    
    Early photographic materials were sensitive to blue light only, but ortho
    materials extended this to green in the middle of the visible region. Green
    is also the colour our eyes are most sensitive to, so this greatly improved
    the tonal rendition of photographs. Ortho materials can be handled using red
    safelights, and are commonly used for special black and white films for
    darkroom use - such as lith film."
    
    Off topic, but how does that "ortho" film relate to right angles?
    
    Alice?
    
    
    

       
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