The instrument is in the form described by Gemma Frisius, with folding meridian, equator, and declination rings and a sliding inner ring with sights.
The meridian ring has a scale of latitude 0 to 90, divided to 10, 5 and 1, and numbered by 10. On the same side is a similarly divided quadrant of polar distance. The other side also carries a similarly divided quadrant of polar distance and the date '1602', executed in a different hand from the rest of the instrument's engraving. The outer rim has 6 holes for the two screws which fix the suspension to the meridian ring. Slots in the suspension plate allow for fine latitude adjustment.
The equatorial ring carries on one face a semicircle of hours 0 to 12 numbered by 1, with each hour subdivided to 20 and 4 minutes. On the other face is a similar semicircle of hours, as well as a shadow square which runs 0 to 12 to 0 divided to 3, 1 and 1/5.
The equatorial ring also carries a zodiacal calendar. The zodiac appears on the outer rim, divided only into the signs with their symbols. The graduations of the ring's hour scales can be used to calibrate the zodiac scale. The calendar is on the inner rim, with each month identified by its initial letter (except 'Ia' for January) and subdivided to 5 and 1.
The declination ring carries a zodiac scale of declination, with symbols for each sign, on one face, along with a semicircular scale of declination, 90 to 0 to 90, divided to 10, 5 and 1, and numbered by 10. The other face carries a circular scale of hours, 0 to 12, 0 to 12, numbered by 1, with each hour subdivided to 20 and 4 minutes. The outer rim of the declination ring names and marks the declination of 20 numbered stars.
The sliding ring slotted into the declination ring carries two sights, each with a pair of pinholes. The inside rim carries a zodiac scale, with each sign represented by its symbol and divided to 5?, with single degree divisions along the edge. The positions of the same 20 stars as on the declination ring are also indicated by their numbers, so that a conversion can be made from a star's right ascension to solar time.
Provenance: Lewis Evans Collection F. 42.