Pixel‐by‐pixel relation between ﹩W( \mathrm{CO}\,) ﹩ and ﹩W( ^{13}\mathrm{CO}\,) ﹩ for the seven Perseus arm clouds observed in CO and ﹩^{13}﹩CO. Top: High‐intensity clouds. Bottom: Low‐intensity clouds. Data above the estimated noise levels are indicated by larger symbols, data below the noise by tiny circles. The line in both plots shows the best linear fit to data above the noise from all clouds.