leucoscope

from The Century Dictionary.

• noun An optical instrument for testing the eyes for color-blindness, devised by Helmholtz.

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

• noun (Physics) An instrument, devised by Professor Helmholtz, for testing the color perception of the eye, or for comparing different lights, as to their constituent colors or their relative whiteness.

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

• noun An instrument for testing the colour perception of the eye, or for comparing different lights in terms of their constituent colour or relative whiteness.