Observational spectrum (blue), after the subtraction of a (very small) solar continuum with the overall fit (red), the sum of CO⁺ (green), ${\text{N}}_2^{+}$ (magenta), and CN (light blue) spectra. The bands appearing around 3800 Å and 4000 Å are CO⁺ bands, respectively (4,0) and (3,0). Top: modeling of the spectrum extracted from plate 2 (30 October 1908). Second: modeling of the spectrum extracted from plate 3 (31 October and 1 November 1908) from the nucleus region, where the signal-to-noise is the best. Third: modeling of the spectrum from the tail region of plate 3 (31 October and 1 November 1908). This one has a larger uncertainty compared to the nucleus region because of the smaller signal-to-noise ratio. Bottom: modeling of the spectrum extracted from plate 4 (28 November 1908).