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We've seen evidence recently that tests for index error using the sea horizon 
yield different ICs for different telescopes on the same sextant. That seems 
very strange. So is it in the specific test involving the sea horizon, or 
does it apply to other tests for index correction (like limb-to-limb tests 
which are generally considered more accurate anyway)? And can anyone think of 
a model that would explain the variability with different scope magnification 
for the standard test? There was a suggestion a "prismatic error" in the 
telescope could be the culprit. How could a prismatic error at any point 
after reflection from the horizon mirror change the angle of contact between 
two objects? If you put a think prism in front of the telescope objective, 
would two objects in contact in the field of view suddenly no longer be in 
contact? I don't think so, but I suppose we should try it out. Also, if 
that's the case, or it's something with a similar origin in the telescope 
optics, can we change it by rotating the telescope? Some sextant telescopes 
cannot be rotated, but others are screwed into a yoke, and presumably we 
could experiment by unscrewing the telescope by half a turn. Is the IC 
unchanged?

-FER



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