  This paper corrects and completes a previous study of the shape of the
extinction curve in the visible and the value of Rv. A continuous
visible/infrared extinction law proportional to 1/{\lambda}^p with p close to 1
({\pm}0.4) is indistinguishable from a perfectly linear law (p = 1) in the
visible within observational precision, but the shape of the curve in the
infrared can be substantially modified. Values of p slightly larger than 1
would account for the increase of extinction (compared to the p = 1 law)
reported for {\lambda} > 1{\mu}m and deeply affect the value of Rv. In the
absence of gray extinction Rv must be 4.04 if p = 1. It becomes 3.14 for p =
1.25, 3.00 for p = 1.30, and 2.76 for p = 1.40. Values of p near 1.3 are also
attributed to extinction by atmospheric aerosols, which indicates that both
phenomena may be governed by similar particle size distributions. A power
extinction law may harmonize visible and infrared data into a single,
continuous, and universal, interstellar extinction law.
