A refractometer instantly reads gravity, in Brix and gravity, of unfermented wort or fruit juice by measuring the degree that light passing through the sample is bent. Unlike a hydrometer, only a few drops are required for a sample. To use, apply 2-3 drops to the prism face, close cover, and look through the eyepiece while aiming your refractometer at a light source.

Our refractometers measure from 0-32 Brix (with an accuracy of +/- 0.2 Brix) and 1.000 to 1.120 SG. They also include automatic temperature compensation for temperatures between 50-86 F, which eliminates the need to consult temperature correction charts.

Please note: the refractometer is not to be used after fermentation has started, light refracts differently through alcohol than it does water and sugar.

BYO also put together a great article here

California Proposition 65

Proposition 65 Warning: This product contains chemicals known to the State of California to cause cancer and/or birth defects or other reproductive harm.