We describe the use of partially overlapping galaxies to provide direct mea
surements of the effective absorption in galaxy disks, independent of assum
ptions about internal disk structure. The nonoverlapping parts of the galax
ies and symmetry considerations are used to reconstruct, via differential p
hotometry, how much background galaxy light is lost in passing through the
foreground disks. Extensive catalog searches and follow-up imaging yield si
milar to 15-25 nearby galaxy pairs suitable for varying degrees of our anal
ysis; 11 of the best such examples are presented here. From these pairs, we
find that interarm extinction is modest, declining from A(B) similar to 1
mag at 0.3R(25)(B) to essentially zero by R-25(B); the interarm dust has a
scale length consistent with that of the disk starlight. In contrast, dust
in spiral arms and resonance rings may be optically thick (A(B) > 2) at vir
tually any radius. Some disks have flatter extinction curves than the Galax
y, with A(B)/A(I) approximate to 1.6; this is probably the signature of clu
mpy dust distributions. Even though typical spirals are not optically thick
throughout their disks, where they are optically thick is correlated with
where they are most luminous: in spiral arms and inner disks. This correlat
ion between absorption and emission regions may account for their apparent
surface brightness being only mildly dependent on inclination, erroneously
indicating that spirals are generally optically thick. Taken as an ensemble
, the opacities of spiral galaxies may be just great enough to significantl
y affect QSO counts, though not enough to cause their high-redshift cutoff.