Knapp (1832- 1911), German-American ophthalmologist and otolaryngologist. Born in Germany, immigrated to New York. Knapp's Rule: A lens placed at the front of the eye (usually 15 mm in front of the eye), as long as the difference between the two eyes is due to the difference in length of the eye axis, a similar size will be produced on the retina. image. This is important for correcting refractive errors with the glasses. But there is some controversy about the soundness of the Knapp rule.