In glasses with plastic lens bows the inner circumference can be enlarged in that the lens bows are heated and thermal expansion dilates the inner circumference. Subsequent cooling permits the inner circumference to shrink again and the lenses are then held firmly regardless of temperature and mode of employment. In the case of metal lens bows the metal section is separated in the region of the hinge and two small metal blocks are soldered to its two ends, this unit comprising the sealing block. The two halves have guides which align when the screw is tightened. This screw traverses the two halves in longitudinal direction. One half has an inside thread whereas the other half has a through boring for the shaft of the screw. When the screw is tightened the two opposing surfaces of the sealing block rest upon one another, which is the maximum tightening point of the screw.
The lenses must, therefore, be precision-ground at the outer edge. For this purpose every manufacturer supplies with every eyeglass model a sample lens whose circumference is copied on the actual lens. Although this copying technique is highly developed and a multitude of copy lenses are maintained in stock, it does sometimes happen that the outer circumference of the lens is ground down too far. For this case thin plastic strips have been developed which are placed into the holding groove of the bow and which are known under the name "Optiker-Trost" [Optician's relief]. The very high requirement of precision is, of course, reflected in the price and the use of Optiker-Trost is only a poor remedy for incorrectly ground lenses.