the manipulating devices known per se for tightening and untightening the screws for securing the housing cover to the fixture housing which are like wise arranged in the assembly cap are also not shown key joints which likewise can be released by so called slide manipulators may also be provided in place of screws in fig2 the same parts as in fig1 are provided with the same reference numerals and repetition is therefore unnecessary in contrast to fig1 in which the guide elements 25 as well as the manipulating device 30 are securely fixed in the assembly cap 18 the relevant parts in the object according to fig2 are arranged inside the assembly cap 18 so as to be movable in a horizontal direction and the housing upper part 20 of said assembly cap is correspondingly made larger for this purpose the lower part of the object according to fig2 from the dividing joint 22 downwards however corresponds in all essential parts to the lower part of the object of fig1 in order that the guide elements 25 and the manipulating devices can be moved horizontally in fig2 a transportation device 48 is provided which consists of a movable frame 49 with transportation rollers 50 which are able to roll along horizontal rails 51 in the present case for every two sets of guide elements 25 or 25a two manipulating devices 30 30a are arranged on the frame 49 the relevant cable winches 31 and 31a can be driven via drive shafts 52 and 52a these are led in a vacuum tight manner through the housing upper part 20 in the operation phase illustrated the frame.