Each of {HEAD ~ the individual discs} can be actuated, with the interposition of {TAIL ~ a stepping mechanism}, via a push button which is adjacent to the viewing windows 
Each of { HEAD ~ the individual discs } can be actuated , with the interposition of { TAIL ~ a stepping mechanism } , via a push button which is adjacent to the viewing windows
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH