NavList:

At a certain moment in space and time I added to George's post:

"I would like to add to your description of my spiegelboog that, as far
as known untill now, this instrument was the first ever reflecting
navigational instrument using a glass mirror. It was invented in 1660
(patent given on September 25th), which was well ahead of Robert Hooke
(1666), Sir Isaac Newton (1699), Thomas Godfrey (1730) and John Hadley
(1731)."

And on the 27th of November (GMT) Frank replied to that as:

 

Of course, the "key" principle applied by Newton, Godfrey, and Hadley was double reflection. The spiegelboog ("mirror bow"?) has only one mirror and does not incorporate that revolutionary double-reflection principle. Or did I miss something?

 

Of course you are right that they incorporated double reflection and Van Breen did not, but Van Breen did make a revolutionairy instrument as I write in my article (and spiegelboog translates as mirror-staff):

=== quote ===
The spiegelboog had a number of advantages over the period instruments. Although it was possible with the back-staff to measure a bright sun backwards, measuring a weak sun was not until about 1670 when John Flamsteed introduced the lens in the shadow- vane. By using a mirror, the spiegelboog made it possible to take a back sight of a weak sun in a hazy sky and in addition to that to take back sights of stars. The spiegelboog was the only navigational instrument with which star observations could be taken backwards before the invention of the octant in 1731. Furthermore the spiegelboog could be used without the mirror for taking back sights of a bright sun and for forward observations when the celestial body would be too low above the horizon to be measured backwards,....
=== end of quote ===

and you need to consider the novolty of the mirror as well:

=== quote ===
...Van Breen put one [a mirror] on his spiegelboog in order to observe the stars for celestial navigation. Another five years later a patent on producing high quality (crystal) glass mirrors in the Low Countries was given for the period of 25 years to Dirck van Cattenburgh, merchant and citizen of Amsterdam.
=== end of quote ===

So a mirror of the quality Van Breen used was quite rare. He even spent eight pages on which he explained in seven propositions how a mirror works, emphasising that this was a novelty. Van Breen probably did get his mirrors from Flanders, the only other glass mirror producers where in Germany and Italy.

The key point of this whole story is that Van Breen was the first who made a navigational instrument that used a glass mirror, while these were not even generally accepted in the scientific community of those days (see the book by C.A. Davids, Zeewezen en Wetenschap. De wetenschap en de ontwikkeling van de navigatietechniek in Nederland tussen 1585 and 1815 (Amsterdam/Dieren, 1986), p. 175.). Of course the double reflecting principle was another big step forward in the evolution of the sextant, still the spiegelboog could be purchased untill at least 1777 and was used on board of VOC vessels next to (or instead of) octants (the latter proof is not yet in the article but I hope to publish it, amongst others, next year). Once you get used to the instrument, observations are as easy to take as with an octant and of about the same quality (on land I have been able to take 16 observations in 34 minutes with a 2.7 arc-minute standard deviation and with an average error of only 1 arc-minute, and that while I started one hour after noon).

Please read the article, it is all in there.

And then Frank continued:

And by the way, welcome aboard! :-)

 

Thank you very much, the group is interesting and discusses celestial navigation and its history more in depth than the yahoo sextants group.

Nicolàs



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