  Given its extraordinary spatial resolution and sensitivity, the projected
Next Generation Space Telescope (NGST) is likely to detect a large number of
high-redshift QSOs lensed by spiral galaxies. Using realistic models for the
QSO and spiral populations, we calculate the expected number density of
detectable QSOs multiply imaged by spirals, and investigate the influence of
various evolution effects on that number density. It is shown that NGST will
probably find of order ten lensed QSOs per square degree at 26th magnitude in
the V and L bands, and that various observable quantities like the total number
density of lensed QSOs in these two bands, the ratio between the number
densities of lensed QSOs in the V and L bands, the fraction of QSOs with more
than two images and so forth can be used to constrain the evolution of the
QSOs, the spirals, the dust in spirals, and the masses of spiral disks.
