1997PASP..109.1193S

begin{tex}Photometry of stars is described that defines a UBV(RI)_MW photometric system, established between 1975 and 1980 using the Mount Wilson 60-inch and 100-inch reflectors. The MW natural system has been reduced to the UBV(RI)_C Cape-Cousins system by linear equations for stars hotter than M0. Strong non-linear color and magnitude equations exit in R and I between the two systems for stars cooler than M0. The non-linearity is due to the change of the effective wavelengths of the red passbands caused by effect of the TiO b ands combines with the substantial difference between red response of the extended S20 cathodes used here and the GaAs cells used by Cousins (1976, 1980a,b), Bessell (1979, 1990), Landolt (1983, 1992), and others for the Cape system. We report (R-I)_MW and (V-I)_MW colors based on a linear extension of the _MW natural system to the red for cool stars with (V-R)_Cape greater than 0.8, (V-I)_Cape greater than 1.8 (spectral types later than M0), as zero-pointed to the Cape system for bluer stars. Color equations are derived between the MW, the Cape, the Kron-Eggen, and Johnson photometric systems, and also the Hubble Space Telescope (V-I) colors as reduced to the (V-I)_C Cape system by Holtzman et al. (1995). The Kron-Eggen and the Cape-Cousins R and I systems differ in their zero points of color and magnitudes, showing that the Kron-Eggen and the Cousins systems are not the same. The appellation of a "Kron-Cousins" photometric system used in much of the current literature is incorrect. Preparatory to determining photometric parallaxes of local higher proper motion M dwarfs in Paper IV, trigonometric HR diagrams are determined in V, (V-R)_MW and (V-I)_MW on the Mount Wilson system using 160 available parallax stars with parallaxes larger than 0.099 arcsec. A first appendix illustrates the difference in the effective wavelengths of the Cape-Cousins, Kron-Eggen, and Johnson systems for cool stars, based on the observed slope coefficients of the color equations that link them. A second appendix contains explicit color equations based on stars in common between Bessell's (1990) definition of the Cape system for the reddest stars and the Kron-Eggen and the MW photometric systems.