[A rare trial eyeglass case with binocular trial lenses].

In this article the author describes a rare trial frame case in the medical history collection of the Ruhr University at Bochum. It has 24 binocular trial lenses which are similar to be early "Nuremberg wire spectacles." The lens power is expressed as the focal length, measured in Rhenish inches. Converted into diopters, the refractive power of the lenses ranges from +/- 1.0 to +/- 8.0 D.spherical. The case also contains six pairs of plane lenses tinted in various intensities of azure blue and four pairs of plane lenses tinted smoke-gray. It contains no astigmatic lenses. Since the metric system, with the diopter as a unit of refractive power, was introduced in the optical industry in 1875, it may be assumed that this case was in use before then. However, the design of the binocular trial frames suggests that they may be considerably older.