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    Re: formula for refraction
    From: Frank Reed CT
    Date: 2007 Mar 21, 20:42 -0700

    Bill, you asked:
    "Here is where I have gone wrong.  Excel is returning the same value
    in C2 as was entered in B2--40.5.
    Any hints or tips?"
    Yes, it turns out that the VLOOKUP function in Excel has a slightly
    different definition from the one in the spreadsheet I was using. Just
    change the third argument in each VLOOKUP from "1" to "2". Then it
    It turns out, though, that you can't use linear interpolation below
    about five degrees if the spacing is single degrees. Instead you have
    to use a spacing of about 0.25 degrees. Thus more typing! And the
    corresponding formula is longer:
     *(1+INT(4*B2)-4*B2) +
    If you have some other dataset where the spacing between the "x"
    values is, let's say, 0.125 units, then replace every "4" in the above
    formula by "8". As a general note, after you've entered a table of
    values like the refraction table in this case, you should graph both
    the numbers and the first differences to make sure you haven't made
    any entry error. A typo will show up as a sudden jump in the graph.
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