We compare 850 \mu m SCUBA images of NGC 891 with the corresponding V-band optical depth predicted from radiation transfer simulations .
These two tracers of dust show a very similar distribution along the minor axis and a reasonable agreement along the major axis .
Assuming that the grains responsible for optical extinction are also the source of 850 \mu m emission we derive a submillimeter emissivity ( emission efficiency ) for dust in the NGC 891 disk .
This quantity is found to be a factor of 2-3 higher than the generally-accepted ( but highly uncertain ) values adopted for the Milky Way .
It should be stated , however , that if a substantial fraction of dust in NGC 891 is clumped , the emissivity in the two galaxies may be quite similar .
We use our newly-acquired emissivity to convert our 850 \mu m images into detailed maps of dust mass and , utilizing 21cm and CO-emission data for NGC 891 , derive the gas-to-dust ratio along the disk .
We compute an average ratio of 260 – a value consistent with the Milky Way and external spirals within the uncertainties in deriving both the dust mass and the quantity of molecular gas .
The bulk of dust in NGC 891 appears to be closely associated with the molecular gas phase although it may start to follow the distribution of atomic hydrogen at radii > 9 kpc ( i.e . > \frac { 1 } { 2 } R _ { 25 } ) .
Using the optical depth of the NGC 891 disk , we quantify how light emitted at high redshift is attenuated by dust residing in foreground spirals .
For B-band observations of galaxies typically found in the Hubble Deep Field , the amount of light lost is expected to be small ( \sim 5 % ) .
This value depends critically on the maximum radial extent of cold dust in spiral disks ( which is poorly known ) .
It may also represent a lower limit if galaxies expel dust over time into the intergalactic medium .