lactoscope

from The Century Dictionary.

• noun An instrument for testing the quality and richness of samples of milk, by their comparative opacity, constructed and operated upon the principle that the richer the milk is in fatty and caseous substances the greater will be its resistance to the passage of light through a stratum of any given thickness.

from the GNU version of the Collaborative International Dictionary of English.

• noun An instrument for estimating the amount of cream contained in milk by ascertaining its relative opacity.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An instrument for estimating the amount of cream in milk, based on its relative opacity.