Before revision:  The classes of equivalence of metamerism is revealed by color-matching experiments, in which observers equalize a monochromatic target stimulus with the superposition of three light beams of different wavelengths (the primaries) by adjusting their intensities.
After revision:  The classes of equivalence of metamerism is revealed by color-matching experiments, in which observers equalize a monochromatic target stimulus with the superposition of three light beams of three pre-set wavelengths (the primaries) by adjusting their intensities.