Plot of the observed k( K − V) ≡ E( K − V)/ E( B − V) vs. the R( V) derived from fits to Equation (5). The 1σ error bars illustrate the correlated errors in the two quantities. The dashed line gives the often-used relation , which is based on van de Hulst's theoretical extinction curve No. 15 (see, e.g., Johnson 1968). An unweighted least squares linear fit to the data is shown as the solid line, .