Distribution of E_B − V and W_CO for four fields mapped here in CO. Each 20′′ pixel in the CO maps is plotted as a separate point. The (red) dashed lines in each panel show the CO emission expected if 25%, 50% and 100% of the gas is in molecular form with a typical value of the W_CO-N(H₂) conversion factor, N(H₂)/W_CO = 2  ×  10²⁰ H₂ cm^-2 (K   km   s^-1)^-1. In each panel a (green) filled diamond is shown at the value given in Table 1 toward the background source.