When working with the stereoscopic microscope it is often desirable to use both hands for continuous manipulation of dissecting instruments or material under observation. In many instances, however, as when using the higher magnifications to study internal organs of insects, repeated focus adjustments are needed. The use of one hand for focus control is then a source of inefficiency and annoyance because it means frequent change of an instrument from its carefully adjusted position in the microscope field. Eyestrain and unsatisfactory performance follow attempts to reduce the number of adiustments made.