Ste. Trombetti has been busily searching the online site of the Spanish National Library, and posting the results on Twitter.
First of these is a view of the Septizonium, the vanished facade of the Palatine, built by Severus at the end of the Appian Way and demolished in the 16th century for materials to build New St Peter’s basilica. This shot is particularly valuable, as it is more or less end-on, from the south, and shows the main structure consisted of two parallel walls, connected at intervals, with the facade on the front. It can be found here (click to enlarge):
Next up is an old photograph of the Colosseum, with a particularly nice image of the Meta Sudans, the fountain just inside the arch of Constantine. Its from here:
A detail shows the fountain clearly:
The same view is shown in an older drawing by Isidro Velazquez, made between 1792-96. Note that in this drawing the Meta Sudans is perceptibly taller, and appears to have a second stage atop the first. It’s from here:
Another image from the same period is an anonymous Spanish painting, “Anfiteatro Flavio, detto il Coloseo”, 1790-99. Here too the Meta Sudans appears taller, and with a bulbous top. From here:
Very interesting to see and compare!