After finishing the cross-staff replica I wanted to reconstruct a navigational instrument that no longer exists: the spiegelboog (mirror-staff). The spiegelboog was invented by Joost van Breen (he sometimes spelled his own name as "Joos', so without a 't') in the 17^th century.¹

On September 25th of 1660 he patented the instrument together with the work in which he described it (see figure 2).

The instrument has similar features as the cross-staff, but uses a mirror on a sliding horizon vane and two vanes on a fixed cross for observations of weak celestial bodies and the sun under hazy conditions. The vanes on the cross can be set at three different positions, simulating the three different vane lengths as known from the cross-staff. The shadow vane could also be used as a cross for very low altitude stars (see figure 10).

In addition to that the mirror can be removed (see figure 4), allowing to use the instrument as a shadow instrument, similar to the cross-staff when used in the Dutch fashion.

Although the instrument has been used for about 100 years by Dutch navigators, none of them survived. Initially they were dedicated instruments, but later vane sets were made available to turn a cross-staff into a spiegelboog. As none are left I made this reconstruction using the information Van Breen left in his book work Stiermans Gemack, which he published back in 1662.

After a year of research, building the instrument took me about two months. Materials used on this reproduction are ebony for the staff and pear wood for the vanes. In addition to that glass, copper and brass are used for the mirror, mirror-frame, pins and springs.

In total two reconstructions were made, the second of which (instrument no. 2) now is in the collection of the Het Scheepvaartmuseum, Amsterdam (inv.no. 2008.0299). On the 22nd of October 2005 I finally presented a paper on the Reconstruction of the spiegelboog at the 'Who needs scientific instruments?' conference at museum Boerhaave in Leiden, The Netherlands.

An article on the instrument has been published in bulletin 101 of the Scientific Instrument Society.¹