I was given this level in 2008 by a former colleague as one of several land surveying instruments and accessories. The instrument was manufactured by Zeiss Opton (level) and Carl Zeiss (parallel plate micrometer), a firm that has its roots in Carl Zeiss Jena.¹ This level came from the West Germany Carl Zeiss factory at Oberkochen and was made there somewhere in the 1960s.

Early automatic levels would level the whole telescope or alidade with visors and existed well before the Ni2. The concept was first published by Michael Butterfield (c. 1635-1724) in the Journal des Sçavans on 6 September 1677, a few years before Christiaan Huygens (14 April 1629 – 8 July 1695) published his version in the same journal on 15 January 1680.² The instrument became widely known and can be found in Bion's 1709 Traité de la construction et des principaux usages des instrumens de mathématique and the 1728 Cyclopaedia. Due to their half-open construction these early pendulum type automatic levels were however quite susceptible to wind.

In 1862 Bauernfeind wrote about them that they could not '...claim any accuracy as even in in still weather their accuracy would never be better than a thousand part of their length...', while due to the use of diopters '... the accuracy of these instruments would hardly be higher than 1:500...'.³ These figures translate to 3.4 and 6.9 arc minutes or 1000-2000mm/km, for which reason he decided not to discuss them in detail.³

As early as in 1790 attempts were made to build reliable automatic levels, like the pendulum telescope by the Bergakademie Clausthal that was able to level a one kilometre double run with an accuracy of approximately 10mm.⁴

In 1944, Hilger & Watts produced the Cowley level, which was based on a mirror attached to a pendulum, had no telescope, and performed less well than the Clausthal instrument.

In the early 20^th century both Heinrich Wild and Carl Zeiss had started experimenting with automatic levels based on vials that refracted the light path, but it took until 1946 that the first automatic level based on this method - by the Russian firm Stodolkjewitsch - found practical application.⁴

The in 1950 introduced Zeiss Ni2 was the first automatic level based on suspended prisms that would level the light path.⁴ It became that successful that in the first two decades already 50,000 instruments were produced.⁴ The instrument would remain in production until around 1990.⁵

The telescope has a magnification power of 32 times and an objective diameter of 40 millimetres. It is 270 millimetres in length and produces an erect image. The cross-hairs form a straight cross with stadia hairs on the vertical hair. In contrast to most other geodetic instruments the cross-hairs only occupy the central 50% of the field of view (see figure 12). The reticle has stadia hairs with a constant of 100.

The compensator allows to level a one kilometre double run with an accuracy of 0.7mm, while the parallel plate micrometer improves this to 0.3mm/km,⁶ which was substantially better than the early pendulum type instruments. The range of the micrometer is 11 millimetres, more than the usual 10 or 5 millimetres that would become usual in later instruments.

Like the Wild NK01 and Wild NK10 the Zeiss Ni2 is equipped with a horizontal circle, which can be viewed through a microscope left of the ocular (see figure 13).

The instrument came complete with its box, shade, and parallel plate micrometer in a separate leather case (see figure 6). Similar to the Wild NA2/GPM3 combination (see figure 11) the plan parallel plate micrometer slides over the telescope and is fastened with a screw (see figure 9).