Two shutter rods are mounted in the base of the spectrophotometer. The left-hand S4
shutter rod is used only when spectrophotometer tests are conducted, and should be pushed all
the way into the instrument when ozone observations are made. The right-hand wavelength
selector rod blocks out light passing either through slit S2 or S4. When this rod is set to position
labels SHORT, only slits S2 and S3 are open so that observations can be made on A, B, C, or D
wavelength pairs. With the wavelength selector rod in the LONG position, only slits S3 and S4 are
open and observations can be made on the C' wavelengths. If C' pair measurements are not
made, then these rods should be locked in place to avoid an accidental movement of the right
hand rod.