  We present CCD photometry in UBVRI of several thousand Galactic field stars
in four large (>1 degree^2) regions centered on diffuse interstellar dust
clouds, commonly referred to as ``cirrus'' clouds (with optical depth A_V less
than unity). Our goal in studying these stars is to investigate the properties
of the cirrus clouds. A comparison of the observed stellar surface density
between on-cloud and off-cloud regions as a function of apparent magnitude in
each of the five bands effectively yields a measure of the extinction through
each cloud. For two of the cirrus clouds, this method is used to derive UBVRI
star counts-based extinction curves, and U-band counts are used to place
constraints on the cloud distance. The color distribution of stars and their
location in (U-B, B-V) and (B-V, V-I) color-color space are analyzed in order
to determine the amount of selective extinction (reddening) caused by the
cirrus. The color excesses, A_lambda-A_V, derived from stellar color histogram
offsets for the four clouds, are better fit by a reddening law that rises
steeply towards short wavelengths [R_V==A_V/E(B-V)<=2] than by the standard law
(R_V=3.1). This may be indicative of a higher-than-average abundance of small
dust grains relative to larger grains in diffuse cirrus clouds. The shape of
the counts-based effective extinction curve and a comparison of different
estimates of the dust optical depth (extinction optical depth derived from
background star counts/colors; emission optical depth derived from far infrared
measurements), are used to measure the degree of clumpiness in clouds. The set
of techniques explored in this paper can be readily adapted to the Sloan
Digital Sky Survey data set in order to carry out a systematic, large-scale
study of cirrus clouds.
