Following is a sample of the information contained on this CD: FIELD OF THE INVENTION The invention relates to a mouth switch arrangement to be mounted on an instrument and to be operated with the mouth of an operator of the instrument. A mouth switch serves to start operations, which are otherwise possibly started with a hand switch, hand key button or foot switch, with the mouth. The use of a mouth switch is particularly advantageous in cases when the operator has no free hand to manually perform the desired operation of a switch. In particular, the mouth switch arrangement is provided for use on an optical instrument, such as a microscope and, in particular, a surgical microscope. BACKGROUND OF THE INVENTION U.S. Pat. No. 3,887,267 discloses a surgical microscope having a mouth switch arrangement comprising a mouth switch for operation with the operators mouth and a rod for mounting the mouth switch on the surgical microscope. On the rod there is mounted a mouth switch support so as to be longitudinally shiftable and lockable by means of an adjusting screw. Due to the mouth switch being longitudinally shiftable on the rod, it is possible to adjust the mouth switch relative to oculars of the microscope, so that the operator can look into the microscope and, at the same time, can operate the mouth switch with the mouth. In situations in which the arrangement of the mouth swicth near the oculars is irritating for the operator, the adjusting screw can be unlocked and the mouth switch can be pivoted away around the rod or can be shifted therealong. Unlocking of the adjusting screw for removing the mouth switch from its operation position is felt to be complicated and inconvenient in practice. In particular, after such unlocking, it is complicated to reposition the mouth switch precisely in the desired operation position again, in which it is operable by the operator in the desired head position, i.e., during observation through the microscope.