Abbé refractometer was the first refractometer to be offered commercially (see refractometer history for more details). It's original design was so successful that even as of today it is over 145 years old, it is still used and copied in new devices.
Abbé refractometer working principle is based on critical angle. Sample is put between two prisms - measuring and illuminating. Light enters sample from the illuminating prism, gets refracted at critical angle at the bottom surface of measuring prism, and then the telescope is used to measure position of the border between bright and light areas. Telescope reverts the image, so the dark area is at the bottom, even if we expect it to be in the upper part of the field of view. Knowing the angle and refractive index of the measuring prism it is not difficult to calculate refractive index of the sample. Surface of the illuminating prism is matted, so that the light enters the sample at all possible angles, including those almost parallel to the surface. If you have seen our page on the critical angle refractometer, you already know that these rays are crucial for the working of the refractometer.