Along the XIXth century there were great advances in the astronomy and
the microscopy fields thanks to the improvements in the image quality obtained
through the objectives of the telescopes and microscopes. This improvement was accomplished
by the suitable combination of lenses with different refraction index reaching
the correction of optical aberrations. In all this process it was essential to measure
with precision the refraction index. Ernst Abbe in 1872 design his first model of refractometer
based on the measure of the limit angle. The great advantage of his instrument
was the simplicity of the design, the compactness, the good precision and the
reduced size which was able to be a portable device. In this work a description, a study
and analysis, including measurements, of an Abbe refractometer from the beginning
of the XXth century are made