Refractometers are instruments to measure substances dissolved in water and certain oils. The refractometer works using the principle of light refraction through liquids. As light passes from air into a liquid it slows down. This phenomenon is what gives a "bent" look to objects that are partially submerged in water. To put it simply, the more dissovled solids water contains, the slower light travels through it, and the more pronouced the "bending" effect on light. Refractometers use this prinicple to determine the amount of dissolved solids in liquids by passing light through a sample and showing the refracted angle on a scale.
The scale most commonly used is referred to as the Brix scale. The Brix scale is defined as: the number of grams of pure cane sugar dissolved in 100 grams of pure water (grams sugar/100 grams H20). Other scales have been developed to measure salt, serum proteins (albumen) and urine specific gravity.