Brosch and colleagues, and other students, investigated the properties of extragalactic dust by concentrating on elliptical galaxies with dark lanes. Modeling the light distribution of the unaffected parts of a galaxy, and subtracting from it the detected light distribution, they derived the amount of light missing because of the dust obscuration. Repeating this with different filters, they found that it is possible to derive the extinction law by the dust in the target galaxy which measures essentially the typical size of dust grains. Since this size is of order 0.1 micron while the typical distance to the galaxies studied is of order 50 Mpc, the ratio of sizes is 31 orders of magnitude.