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George, you wrote:
"He goes on, to refer to "the mean of
readings as one image is brought alternately up and down" and to "the point
of tangency of imperfect images", which reinforces that view. In which case,
that 5' range would comprise two whole disc-diameters of a star image, and I
would expect a careful observer to be able to assess the index error to
within an arc-minute or so. It would allow us to assess the resolution of
his telescope as just "poor", rather than as "grotesquely-bad"."

Yes, that's possible. His technique might also be explained by the standard 
definition of diffraction-limited resolution. The traditional formula (which 
he uses in one form later in the paper when he brings up his "theoretical 
resolving power") is defined by placing the center of the Airy disk for one 
point source of light on the first minimum of the Airy disk for the second, 
which is similar to placing two "star images" side-by-side. He may have 
extrapolated that definition and tried to do the same with the "blurry" 
images of stars in his telescopes. 

-FER



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