Light obeys an inverse square law: observe the headlight of a car from a fi xed distance and then let the car travel four times that distance: the intensity of the headlight's beam will have dropped by a factor of sixteen (the square of four is sixteen). If one assumes that open clusters of stars may be categorized into certain subclasses with fi xed diameters (in light years, for example), Trumpler showed that the distances to the further clusters were systematically too large. The reason is this: foreground dust between a cluster of stars and the observer fools the astronomer in thinking that the cluster is dimmer (and therefore, further) than it actually is. Distances derived from the inverse square law for clusters obscured by foreground clouds of dust are too large. A ship sailing in thick fog may actually be relatively nearby, but it may appear to be much farther away on account of the dimness of the lights on deck as seen through the fog!
Once astronomers had accepted a dusty Milky Way, the question beckoned as to the amounts of dust in spiral galaxies external to the Milky Way. Prior to 1994, the prevailing opinion amongst astronomers was that our Milky Way was particularly rich in cosmic dust — in technical terms, its ratio of “dust-to-gas” mass was believed to be ten times higher than that for other spiral galaxies. We shall see the raison d'etre behind this argument, presently.