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George,

Thank you for tabulating my results. I was too lazy to do so.

You wrote

"The surprise, to me, is not in the difference in the scatters, but in the
difference between the mean values of zero-error."

Yes, these different results for the mean index error is what surprised me and 
led me to do the second series of naked eye observations. If we write down 
the 95% standard errors for the means (1.96 x SD/root n), which I should have 
calculated, we get:

naked eye series 1     1.457 +/- 0.164   95% range 1.62  -  1.306
naked eye series 2     1.30  +/- 0.148             1.45  -  1.15
x 4 scope              0.743 +/- 0.067             0.81  -  0.68
x 6 scope              0.593 +/- 0.046             0.64  -  0.55

The differences of the means are clearly real at the 95 % confidence level. 

I suppose now I should do a whole new series and also look for another 
instrument in which there is no paint line on the horizon glass. I think I 
have an old Tamaya which has just air in place of the plain glass, but of 
course its at the bottom of a pile of boxes, it's getting late and my brain 
is hurting.

Frank,

In comparing the scatters, I wonder whether it's the variances (SD squared)we 
should be comparing, rather than the SDs? My statistical education, such as 
it was, is too far away and anyway, I have little mathematical intuition. 
You,  on the other hand, seem to have lots. Perhaps you can comment.

Bill B 

You have reminded me that I did not say how I handled the micrometer. I always 
brought the reflected image down to the direct image, in the direction of 
increasing reading. In most sextants this places load on the thrust bearing 
of the micrometer rather than on the axial pre-load spring and in theory at 
least, gives more consistent results. The US BuShips Mk II has a left hand 
thread and I think the load comes the other way around, but it's at the 
bottom of another pile of boxes... In the SNO-T it doesn't matter, as long as 
it's always the same way.


Bill Morris




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