NavList:

David, 

The sea view is defintely to the left, as shown for my instrument and C Plath's version in Friedrich Jerchow's history of Plath, "From Sextant to Satelllite Navigation." I have some problem with the illustration that shows the sun super-imposed upon the bubble. This is not possible with the telescope in my instrument as it is a Galilean and the objective only "sees" its own bit of the field, so the the sun is well out of focus in the central part of the field. It is possible that the illustration was for the peep sight, though I note that the telescope shown on the right hand opening of the book has an eyepiece with two lenses, suggesting it is an inverting telescope, in which the two lateral halves would "see" the whole field of view. My reconstruction of "Byrd's" sextant also has a Galilean with an auxiliary half-lens and the same disadvantage, while Willson's AH attachment has an inverting telescope, but as he uses a different system with a circular bubble, in which a real image of the bubble is required.

This type of sextant is far from easy to use and why Coutinho did not use a circular level with a beam splitter is beyond my comprehension. I suspect that it was useful in airships, which could be stopped while taking observations. I assume then that turbulence would be less of a problem. Can any ballooners comment?

I hope to be describing the sextant in detail within a very few days as, having broken my last 0.8 mm drill this afternoon, I am waiting for the arrival of replacements in order to proceed with making a new detent for one of my chronometers. The photos are ready, so the text will soon follow.

Bill Morris

Pukenui

New Zealand