Abstract: In eyeglass lenses which have a center thickness greater than the edge thickness in at least one cross section and different dioptric powers in different sections, round-blanks lead to ground eyeglass lenses which are of relatively high weight. The weight of such an eyeglass lens, and thus its center and edge thickness, is reduced if the blank is imparted a substantially elliptical edge curve. In order to reduce cost and expense, the dioptric range of the series of eyeglass lenses is divided into a predetermined number of ranges and a given type of edge curve is associated with each range. Within each range there are a number of centrally similar edge curves of different height and width for such type of edge curve. All edge curves are so selected that the eyeglass lens blanks provided with them have an edge thickness over the elliptical portion of their periphery which varies by less than 1.5 mm. An easily operated device is provided in order to select the optimal blank for mounting in an eyeglass frame.

Abstract: A device for rapidly demonstrating the darkening action of phototropic objects, particularly eyeglasses. The device has a housing or casing provided in its lower part (6) with a hollow space (7) which is placed over the objects to be demonstrated (8, 9) while such objects lie on any suitable horizontal supporting surface such as a table or desk. These objects are uniformly illuminated by a flash tube emitting light in the UV-A range and in the visible range. Upon each actuation of the circuit controlling the flash tube a plurality of flashes which succeed each other at a predetermined time interval is automatically given off.