Abstract: An operation microscope includes an illuminating device and an interference filter in the illuminating beam of the illuminating device. The interference filter is suitable for incorporation into the operation microscope. In order to prevent the yellow tinge that is present in long-known UV interference filters, the interference layer applied directly to the carrier substrate is a layer between 8 and 12 nm thick of yttrium fluoride, thorium fluoride, lanthanum fluoride or cerium fluoride.

Abstract: A thin film of gallium oxidized with oxygen is produced by reactive vapor depositing gallium in a vacuum with oxygen followed by tempering. The refractive index (n) lies in the range of 1.2 to 1.3.

Abstract: The invention contemplates a circular spectacle-lens blank which is optically finished on both its front and back surfaces and which is characterized by locally hardened registering central areas of both surfaces. The hardened areas are sized to be universally included within the spectacle-frame profile of all frames within a given variety of frames, and the outer annular areas are unhardened and of such effective radial extent as to embrace all of such profiles within said variety, so that each finished and hardened blank is adapted for later edging to the profile of a later selected one of the frames within the given variety. Various methods are described for creating the indicated locally hardened regions on the otherwise-finished lens blanks.