We determine and tabulate A sub( lambda )/A sub(K), the wavelength dependence of interstellar extinction, in the Galactic plane for 1.25 mu m less than or equal to lambda less than or equal to mu 8.0 mu m along two lines of sight: l = 42 degree and 284 degree . The first is a relatively quiescent and unremarkable region; the second contains the giant H II region RCW 49, as well as a "field" region unrelated to the cluster and nebulosity. Areas near these Galactic longitudes were imaged at J, H, and K bands by 2MASS and at 3-8 mu m by Spitzer for the GLIMPSE Legacy program. We measure the mean values of the color excess ratios (A sub( lambda ) - A sub(K))/(A sub(J) - A sub(K)) directly from the color distributions of observed stars. The extinction ratio between two of the filters, e.g., A sub(J)/A sub(K), is required to calculate A sub( lambda )/A sub(K) from those measured ratios. We use the apparent JHK magnitudes of giant stars along our two sight lines and fit the reddening as a function of magnitude (distance) to determine A sub(J)kpc super(-1), A sub(K)kpc super(-1), and A sub(J)/A sub(K). Our values of A sub( lambda )/A sub(K) show a flattening across the 3-8 mu m wavelength range, roughly consistent with the extinction measurements derived by Lutz and coworkers for the sight line toward the Galactic center.