spring 73 with the apparatus 20 in its upward position the movable shutter 74 can be closed as is apparent the movable shutter 74 lever 94 and pressure bar 88 must be operated in the correct sequence for the exposure of the film to be properly completed to reduce the chances for operator error an interlock system has been incorporated into the exposure apparatus 22 to prevent an incorrect sequence of operations the interlock system is best seen by reference to fig7 10 the shutter handle 76 projects from the side of the lower housing 64 in a position to contact a side of the translation control lever 94 in the manner illustrated in fig9 the control lever 94 cannot be rotated to translate the reaction test apparatus 20 downwardly unless the shutter handle is moved to the outward position to align the openings 70 and 78 the shutter handle 76 is attached to the shutter 74 which cannot be moved unless the upper housing 66 is closed to the lower housing 68 to form a light tight seal therewith due to an interlock between the two parts when the shutter handle 76 is operated to open the shutter 74 a notch 110 in the handle 76 engages a tab 112 on the upper housing 66 to lock the upper housing 66 closed.