OPSIOMETER

(Yunanca köken — orijinale bakınız.) An instrument for measuring the extent of the limits of distinct vision in different individuals, and consequently for determining the focal lengths of lenses nece ary to correct imperfections of the eye. A contrivance for this purpose, by M. Lehot, is described in the Annates des Sciences d' Observation for June 1829, and in the Notes by M. Quetelet to the French translation of HerscheVs Treatise on Light. Its principle depends on the appearance presented by a straight line placed very near the eye, in the direction of its axis; and the principle is carried into practice by placing a thread of white silk on a narrow rule covered with black velvet, and furnished with a suitable apparatus for marking the exact points at which the thread begins and ceases to ba distinctly seen, when held in a certain position with respect to the eye. An instrument for the same purpose, on a different principle, had formerly been suggested by Dr. Young. [s. 860]