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Brad,

Are you sure that the shade is clear glass? I wonder whether it might not be a 
cylindrical lens, used as an astigmatiser to draw out the image of a star 
into a line.

In a previous post, of your Heath and Co sextant, you wrote :

"At this juncture, I would like to have the arc calibrated but as we all know, 
those services just don't exist anymore.  I keep hoping Frank will determine 
the economic feasibility of his calibration methodology alluded to in earlier 
posts. But until then, I am forced to use the National Physical Laboratory 
record of 1921 of my sextant, which shows ZERO minutes and ZERO seconds for 
every reading from 15 degrees to 120 degrees. Naturally, the device should 
have worn over the years.  Maybe Bill Morris can comment on the evenness of 
wear over the years.  Uneven wear would contribute to an eccentricity and may 
throw that unbelievable calibration certificate out."

To try to answer your question, I have recalibrated my Hughes and Son vernier 
sextant of the same period. It is a ladder frame instrument and has a Class A 
NPL certificate dated 1920, showing zero error every 15 degrees to 120 
degrees. In a previous posting, I have commented that I would not expect much 
wear, and the zero error must take into account that the instrument cannot be 
read to  better than 10 arcseconds, probably rather less for most of us with 
older eyes. The graduations have nice sharp edges, without chatter marks in 
the depths, and are as easily readable as nearly any other vernier scale i 
have come across. I read them with the Ramsden magnifier provided and lots of 
blue-white light.

Do not attach too much importance to my having given the error to the nearest 
second. This is justified probably only for the 15 degree reading which, on 
account of the method used, is in effect an average of 24 readings of the 
sextant. A better interpretation of the results might be "broadly in 
agreement with NPL certificate." 

Sextant Reading     Error
    Degrees           arcseconds

    0           0
    15          -4
    30          +7
    45          +9
    60          +3
    75          -1
    90          +6
    105         +3
    120         -7

Bill Morris
Pukenui
New Zealand




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