The lengths of the links comply with the conditions: A͞B=D͞C and A͞D=B͞C. The links of the mechanism are arranged symmetrically with respect to axis y-y. Each half 3 of the diaphragm with levers 1 and 2 forms a parallel-crank linkage ABCD. When lever 1 turns about fixed axis A, lever 2 is turned and the action of pin 4 sliding along slot 5 turns the second lever 2. This retracts the two halves of the diaphragm forming a rectangular slit.
$1472$LG,FD$