The skilled optician should be practised in producing single prisms and lenses of high quality, a process which, like glass blowing, although very simple requires much skill and experience. In the mass production of lenses, polishing processes have developed in two directions. The first aims at increasing the number of polishing spindles which can be attended by a single workman. In the second, in use in other factories, particularly in some making large numbers of lenses of high quality, carefully designed machines are used to secure that the best conditions for good polishing are automatically maintained. In 1945 it was still considered in Germany that moulded lens blanks were insufficiently homogeneous for the highest class of optical instruments, and it was therefore customary, for the best work, to cut the blanks from slab glass. The optical parts for binoculars, however, were generally made from mouldings.