Brief history of refractometers
Ernst Abbé designed his refractometer in 1869, but it didn't became commercially available from Carl Zeiss before 1881. For the next almost 40 years Carl Zeiss was the only producer of Abbé refractometers. The device was slowly changing - while the general principle was (and is up to today) still the same, new models were finetuned to increase accuracy of the masurements. One of the important changes was introduction of the temperature jacketed prisms, which took place around 1893.
Since the late nineteenth century refractometry was one of the main techniques used in chemical analysis to determine concentrations of solutions and as an aid in the identification of unknown substances. In practice it is not possible to determine refractive index with sufficient precision for the unique identification of the substance - that's not different from boiling and melting points - but similarly to phase transition temperatures refractive index is an important property of the substance that facilitates final identification.
The most versatile refractometer for laboratory use is the Abbé refractometer and its variations. Abbé refractometer provides a quick and easy way to determine the refractive index and dispersion of liquids and solids. Abbé refractometers are used in chemical and food industry, and in medical laboratories. Operating principles of the device - based on the measurement of the critical angle - are described on the Abbé refractometer page.
Carl Zeiss was the first and for many years the only manufacturer of Abbé refractometer. In 1874, Ernst Abbé published booklet Neue Apparate zur Bestimmung des Brechungs - Zerstreuungsvermögens und fester Körper und flüssiger, which described the construction and principle of operation of the refractometer he designed five years earlier. Basic build of the original apparatus was identical with the modern versions of Abbé refractometers, the only major difference - one having no connection with the instrument optics - was the lack of water jacket allowing for measurements at constant temperature. Refractometer was initially produced only on request, for special customers and product did not appear in Carl Zeiss catalogs before 1881 (but even then it landed between microscopes).
In 1888 Carl Pulfrich designed and described another device, also making use of the critical angle, however his design was significantly different from the Abbé refractometer. Initially, the refractometer designed by Pulfrich was made by Max Wolz in Bonn, but after Pulfrich joined Carl Zeiss his refractometer became one of the Zeiss products. Since that time the device is known as the Pulfrich refractometer.
Although the two main refractometers - Abbé and Pulfrich - were able to meet needs of majority of analytical laboratories, work on better devices never stopped. New designs included devices suitable for use outside the laboratory (such as immersion refractometers, offered by Zeiss since around 1900) and devices providing better precision than typical value of 0.0001 attainable with Abbé refractometers.
Situation changed with the end of WWI, when other producers started to market Abbé refractometers. There were several reasons for that. As a result of the First World War the Germans were forced to pay war reparations. Some of them have been paid by the transfer of patents and designs of equipment produced by German industry to US and UK firms. That happened to optical devices produced in factories Carl Zeiss Jena, in effect Abbé and Pulfrich refractometers became available from Bausch & Lomb (US) and Adam Hilger (UK). At the same time many firms that during war were involved in production of optical instruments for army were looking for new markets, and refractometers fit their manufacturing profile. That led to production of many new models, mostly based on similar designs.
Zeiss factory in Jena produced refractometers until the destruction by the Allied bombings during World War II. Up to this moment Abbé refractometer was modified many times, small changes slowly improved user experience with the device, its resistance to the laboratory environment and accuracy of the measurements.
At the end of World War II Germany was split, same happened to Carl Zeiss. Factory in Jena was in the Russian zone, the occupying Russians relocated most of the existing Zeiss factories and tooling to the Soviet Union, together with documents and designs. The western part of the company therefore began work on a new device. New refractometer used a single telescope both for positioning of the border between light and dark areas, and for reading the refractive index, which made it much easier to use.
Much more on the history of refractometers here.