Survey sextant that was once purchased by Volker Stevin Dredging ('Baggermaatschappij') for use in hydrographic work. It is equipped with a prism instead of a horizon mirror. This is the only sextant I know that uses a prism construction like this. In all my other sextants with a prism, the prism is separate from the horizon mirror and can be detached. On this sextant both the index mirror ánd a 90 degree additional angle can be seen at the same time. The horizon can be seen in the 'regular' way, by looking over the prism. This means that the two bottom images are respectively right object (middle) and the left '90 degree' (bottom) or that the two top images are respectively the horizon (top) and right object (middle).

Apart from that, the brochure states that this instrument does not exist in the configuration that is in my collection. In Dutch, it states 'Uitsluitend voor gebruik zonder Pentagon prisma ook leverbaar met een sexagesimale verdeling' or, translated, 'Exclusively for use without Pentagon prism also available with a sexagesimal scale'. As this has both a sexagesimal scale ánd the prism it should not have been supplied to Volker Stevin.

My guess is that Volker Stevin, being one of the major Dutch dredging companies at the time 'convinced' Observator to supply them with this version. Volker was started in 1854 by Adriaan Volker; in 1978 they merged with the Stevin group to become the largest construction company of the Netherlands. Much of the dredging was done abroad of which this sextant shows some testimoney as it still contains the labels from the last job it was on; probably Tunesia considering it was transported by TunAir. Via various take overs and mergers the dredging activities became part of van Oord; still one of the worlds largest dredging companies.

More information / other collections:

Wikipedia

Dredgepoint

van Oord