Summary of the prism observations. Panel a; S/N of each of the 162 individual integrations. The horizontal dashed line separates the bottom 114 rows with the 3215 integrations from the 1210 integrations above. Panel b; spectrum of each of the individual integrations. A feature at the location of C III] is clearly seen. Panel c; median and 16th–84th percentile of panel b, showing again the presence of C III], and the possible presence of [O II]. Panel d; comparison between the noise estimated by the data reduction pipeline (black) with the noise estimated from the dispersion of the integrations (grey, calculated under a Gaussian-noise assumption). Panels e–h; comparison between 3215 (sand) and 1210 (cyan), showing that C III] is seen in both programmes; in contrast, O III] is not seen in 3215 (the deepest of the two surveys); an emission line close to the position of [O II] is seen in both surveys, but at different wavelengths; [Ne III] is seen in 3215, but has an unconvincing line profile in 1210.