The instrument consists of a square brass plate with a revolving disc which has one quadrant partly cut away. Each corner of the plate is marked with a letter: 'A', 'B', 'C', or 'D'.
The plate has four circular scales, the first two lying beyond the disc. The outer scale is a circular shadow square, identified in each corner as 'VMBRA RECTA' and 'VMBRA VERSA'. Each quadrant is divided 0 to 12 to 0, divided to 1 and 1/5, and numbered by 1. Inside this there is a scale of degrees 0 to 360, divided to 10, 5, 1, 1/2, 1/4 and numbered by 10.
The other scales on the plate are only visible through the open quadrant of the disc. There is a wind rose with 32 directions named in Dutch (with the four cardinal directions also named in Latin) around which is a corresponding circular scale 0 to 32, divided to 1, 1/2, 1/10 and numbered by 1. This scale is matched by a simpler version towards the centre of the plate, which has 32 parts alternately hatched and without any graduation.
The revolving disc carries hour lines for a horizontal dial with an adjustable gnomon, for latitudes 30 to 63?, with a declination line for each degree of latitude. The hour lines are identified at the edge of the disc as IIII to XII to VIII, and a distinction between 'Antemeridianae horae' and 'Pomeridianae horae'. Across the declination lines and towards the centre of the disc the hour lines are graduated 4 to 12 to 8. Each hour line is marked by an arrowed line, each half hour by a line and the quarter hours by incisions on every third declination line.
The type of gnomon and method of fixing is not clear, but a string gnomon seems the most likely possibility.
The disc has a pointer at each of the cardinal points which can be read against the shadow square and degree scales. The open quadrant of the disc is towards south, where the night hours would be, and there are two further pointers in the direction of south to be read against the wind rose scales.
In the centre of the disc is a compass surrounded by a ring with the cardinal points named in Latin. The compass bowl is 73 mm in diameter and has a scale of degrees 0 to 90 to 0 to 90 to, divided to 10 and 2 and numbered by 10. There is also a cross of direction labelled with the four principal Italian winds, and a magnetic variation of about 9? east is marked. The needle and glass are missing.
At its east and west points the disc has a block (one marked 'I') onto which sights would have been slid.
On the reverse of the plate is a cylindrical socket to receive a post for mounting the instrument horizontally. There are also two suspension rings fixed on opposite edges.
The reverse also carries a (presumably abandoned) quadrant. Divided along two edges are a decimal shadow square 0 to 100, divided to 10, 5 and 1, and numbered by 10; a scale of degrees 0 to 90 divided to 10, 5, 1, 1/2, and 10 minutes, and numbered by 5; and a shadow square 0 to 12, divided to 1, 1/5, 1/25, and numbered by 1. There is a hole in the appropriate corner where an alidade may once have been fixed.
See K. van Cleempoel, "A simple theodolite and sundial attributable to Gualterus Arsenius", Sph?ra: The Newsletter of the Museum of the History of Science, Oxford, issue no. 7 (Spring, 1998), p. 6.