Optical measuring instrument (refractive index meter) with which the concentration of liquids or solid bodies can be determined by means of light refraction. A variant of such a device was developed by the German physicist Ernst Abbe (1840-1905), who was head of the observatory in Jena and owner of the Zeiss company. In viticulture, this makes it possible to determine the sugar content in the form of must weight and thus potential alcohol content already in the grapes in the vineyard or in the grape must. In a drop of grape must enclosed between two prisms, the light is diffracted according to the sugar content and the value is usually displayed on a scale in several common units of measurement (e.g. Brix, KMW, Oechsle). The more concentrated the sugar solution, the more the light is refracted. This is used to determine the degree of maturity or physiological ripeness as a decision-making aid for the ideal time to harvest the grapes. In order to obtain a representative value, at least 100 berries must be taken from different zones of the grapes and necessarily also from several bunches from different locations in the vineyard. These berries are crushed together in a container and then some juice is taken for the measurement. The must weight development is determined by several measurements on different days in succession. For other methods of measurement, see analytical testing.