  Parallel slit-slit devices are commonly used to measure the transverse
emittance of a particle beam, selecting a portion of the beam with the front
slit and measuring the angular distribution with the rear. This paper
calculates the effect of finite slit sizes on measured emittance and Twiss
functions in the case of Gaussian spatial and angular distributions of the
oncoming beam. A formula for recovering the true emittance from the measured
values is derived.
