star. The USNO-A2.0 catalog contains $>$5 10$^8$ objects ($\sim$12,750 per square degree) and is based on scans of the Palomar Sky Survey (PSS) plates produced with the Precision Measuring Machine (PMM). The catalog is an improvement over the version 1.0 both in astrometric accuracy and in photometric precision. The photometric accuracy is probably not better than $\sim$0.15 mag, but the depth of the catalog is considerable, as it reaches 20-22 mag (color-dependent). It can, therefore, serve as a source of stellar objects with which one can test the assumption of foreground extinction.

We extracted objects in a number of $1^{\circ}\times1^{\circ}$ regions from the USNO-A2.0 catalog. The extraction locations are listed in Table 1 and correspond to some FIR-bright regions (where we expect a higher density of extinguishing dust) or to some FIR-faint regions (which should be $\sim$transparent). We produced Wolf diagrams for each location, and show these in Figure 2. The Wolf diagrams plot the cumulative distribution of stellar magnitudes against magnitude, and the signature of total extinction in such a plot is a step-like deviation, to fainter magnitudes of the cumulative star counts, from the pattern set by the brighter (and closer, on average) stars. The diagrams do not 