THE MASKING SHUTTER (chopper) lever is the middle lever projecting through the top of
the optical system housing. The masking shutter blades are operated by this lever to shape the projected spot to
a rectangle, strip spot, or dousing out to full cutoff. The disengaged (full open) position of the masking shutter
lever is to the extreme right (viewed from rear). Varying degrees of masking to complete cutoff are obtained by
moving the lever to the left