An automatic refractometer comprising a photosensitive device having a relatively narrow dynamic range in the form of a linear scanned array including a plurality of photoelectric elements each providing an output pulse during a scan and the amplitude of each pulse being determined by the amount of illumination of the corresponding element by incident light, an optical system for directing light onto the array in a manner such that the particular photoelectric elements of the array which are illuminated by the light are determined by the index of refraction of a light transmitting substance placed in operative association with the optical system, a circuit for converting signals from the array into digital signals containing information as to the amplitudes of the signals from the array, a digital processing circuit for storing respective signals from reference and sample substances placed in operative association with the optical system and for computing the index of refraction of the sample substance by means of a comparison of the stored reference and sample information, and apparatus for providing a read out of the computed result. The digital processing circuit also calculates the percent solids in the sample substance, and the circuit also includes a plurality of channels for containing information to provide different interpretations of the index of refraction computed thereby. The circuit for converting array signals into digital signals comprises a peak detector circuit for detecting peak amplitudes of signals obtained from scanning the array and an analog-to-digital converter for providing digital signals containing information as to peak amplitudes of the array signals. There is also provided arrangements for measuring the temperatures of the sample substance and comparing to a reference for applying a temperature correction to the computed index of refraction, monitoring and regulating the temperature of the component of the optical system to which the sample substance is exposed, and monitoring and regulating the intensity of light incident on the array.