eriometer

from The Century Dictionary.

• noun An optical instrument for measuring the diameters of minute particles and fibers from the size of the colored rings produced by the diffraction of the light in which the objects are viewed.

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

• noun (Opt.) An instrument for measuring the diameters of minute particles or fibers, from the size of the colored rings produced by the diffraction of the light in which the objects are viewed.

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

• noun An optical device used to measure the diameters of fibres such as wool