The dust content of spiral galaxies is an important, and poorly
 known, property. Optically backlit galaxies offer a direct way
 to measure the extinction through spiral disks, and give results
 suggesting that the dust is clumped, which is crucial to
 the overall effects of dust on the starlight. ISO measurements
 offer the opportunity for a classical test for the degree of
 clumping, via measurements of the same dust population in emission
 and in absorption. We propose ISO observations at 12,50,100, and
 200 microns of the four most appropriate galaxy pairs, in which a
 smooth early-type galaxy shines through the disk of a reasonably
 symmetric foreground spiral. As long as the 200-micron band has an
 important contribution from the mass-dominant part of the dust
 distribution, we will be able to derive any of several indices of
 dust clumping, for input to more realistic models of the transfer
 of stellar radiation within galaxies.