NavList:

Gary, you wrote:
"I am attaching an excerpt from the book "Precision Astrolabe" that describes the Coutinho sextant and other sextants adapted to aviation use."

Thank you. That's a rather rare book. Copies on Abebooks.com are selling for about $65 and up. Could I ask you to post pages 192-193? They don't seem to have made it into your sextants.pdf file.

This book, as well as another which I found through google, stress that the key feature that was novel at the time was that the Gago Coutinho bubble horizon employed a spirit level which was curved in just the right ratio so that if the sextant is pitched up and down (rotated about an axis perpendicular to the sextant frame and passing through the eye-end of the sighting tube or telescope) the image of the Sun or star and the bubble remain together much the same as with respect to the sea horizon in a standard marine sextant. This was standard in later bubble sextants, but back then it was apparently a big deal. Also, the diagrams in the pages you posted clear up one other puzzle. In photos of his original prototype and also of the production Plath versions, the housing for the bubble and the mirror appear to be behind the mirrored part of the horizon glass, and we can even see that the usual horizon shades are available for the clear part of the horizon glass. The bubble housing does not block them (not completely at least). According to the book, the horizon mirror of the Gago Coutinho type of sextant was unique in that the horizon glass was divided into three parts. Only a small central section was fully mirrored. The section away from the frame was clear and allowed a view of the sea horizon if available. The section near the frame was also clear and allowed a clear view of the bubble. Of course, all three sections could reflect the image of the celestial body.

-FER

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