The high-fidelity solar spectral atlas prepared by Delbouille et. al (1973, Liege atlas, Atlas photometrique du spectre solaire de {lambda} 3000 a {lambda} 10000, 1973apds.book.....D) and the atlas by Neckel (1999, Hamburg atlas, 1999SoPh..184..421N) are widely recognised as the most important collection of reference spectra of the Sun at disc centre in the visible wavelength range. The two datasets serve as fundamental resources for many researchers, in particular for chemical abundance analyses. But despite their similar published specifications (spectral resolution and noise level), the shapes of the spectral lines in the two atlases differ significantly and systematically. Knowledge of any instrumental degradations is imperative to fully exploit the information content of spectroscopic data. We seek to investigate the magnitude of these differences and explain the possible sources. We provide the wavelength-dependent correction parameters that need to be taken into account when the spectra are to be compared with synthetic data, for instance.