Although not a standard like the inch, yard or chain in the collection, this instrument has for a long time defined a different kind of standard here in the Netherlands. It was designed by TU Delft Professor Roelofs in order to test the circle divisions of theodolites.

A scientific approach of theodolite circle testing started in the early 20^th century with TU Delft professor Hk.J. Heuvelink. He wanted to find the random and periodic errors in the divisions of theodolite circles and published his ideas in 1913 together with a list of 42 theodolites he tested with his method.¹. He used two collimators at a mutual horizontal angle of 45 degrees and measured them 36 times, divided over nine circle positions which all were measured four times.¹

As theodolites were not yet read by coincidence, as would become standard after Heinrich Wild designed the 1924 Zeiss Th1 readings were taken from two opposite circle reading telescopes. The Th1 was the first optical theodolite to be tested using Heuvelink's method, which was done by Wim Schermerhorn in 1924 and published in 1925.²

Heuvelink's method was used for circle testing until the 1960s. In 1965 professor Roelofs published an article about a new way of circle testing.³

Instead of the usual two directions Roelofs used a 'bundle' of four directions that were observed at regular intervals along the theodolite's horizontal circle. In his article Roelofs explained that this could be done with simple targets as long as they were at least 5 metres away from the theodolite.

Using collimators was however preferred and for that Roelofs designed a circle testing instrument which was subsequently built by Breithaupt and sold under the name PRURO. The PRURO has been used for years by Roelofs and others at the TU Delft here in the Netherlands.

In 2011 it was offered to my collection, but with our new house under construction it had to wait until that was finished. Knowing that it would become part of the collection a dedicated foundation was created in my workshop to rest the instrument on. In December 2013 I finally collected this well over 100 kg instrument. In order to move it through the building and into my car we had to dismantle it completely. Finally at home a stand had to be made to rest the instrument on, which I made of steel and painted a slightly lighter than the PRURO.

The PRURO has 4 collimators (focal length 600 mm, objective aperture 60 mm) which are arranged for reference angles of 0g, 15g, 37g and 90g; according to the height of the telescope tilting axis of the theodolite their height may be adjusted between 155 and 300 mm.⁴

The first instrument I had on it was my geodetic Wild T3. A full set of observations was made with it, but I still need to create the software to analyse it.