Member 1, sliding in stationary guide b, has head d whose knife edge enters V-slot f of member 2. Member 3 , with V-slot k, has frame 8 ending in hand 4. Rocking prism 6 has knife edges at each end which enter V-slots k and m of frame member 3 and stationary head e. Straight-line motion of member 1 is transmitted through members 2 and 3 to hand 4. Spring 5 holds members 1 and 2 in constant contact. Frame member 3 can be adjusted with respect to member 2 by screws 7 to change distance a between member 1 and head e.
$474$SL,M$