One of the most important applications for stereoscopic microscopes is as operating microscopes in microsurgery. When using the operating microscope for this purpose, the type of illumination and the possibility of simultaneous observation, i.e. the simultaneous use of the operating microscope by two observers or use by one observer and simultaneous observation on a television screen, are important factors. As surgical procedures become more and more complicated, the surgeon must increasingly rely on an assistant for certain steps in the procedure and for general cooperation. With respect to instruments, it is mandatory that the assistant has the same or almost the same opportunities for working and observing as the principal surgeon.
It is known to provide an operating microscope with two complete stereoscopic optical paths at 90.degree. in such a manner that they share a common housing and a common magnifying objective. See, for example, U.S. Pat. No. 4,138,191, granted Feb. 6, 1979, to Peyman et al. One disadvantage of this solution is that the instrument becomes very cumbersome, which prevents compact design with enclosed illumination and makes it difficult to provide zoom lens systems for all four light paths.