The upper and lower panels show the absorption spectra of the H₂CO 2₁₁ → 2₁₂ transition toward compact source A of the gravitational lens system B0218+357 produced from our 1995 August and September data, respectively. Portions of the 1995 August spectrum that are potentially affected by interference are drawn as a dotted line. The velocity scale assumes a redshift of 0.68466 in the heliocentric frame. Channel spacings are 3.4 and 1.7 km s⁻¹ for the August and September spectra, respectively.