LotR -- Book 5 - Chapter 02 - The Passing of the Grey Company

[Lalwendë]

Originally Posted by davem

The only thing that springs to mind is that the Palantiri were not simply a means of seeing what was happening in the present moment, but also, in some way, retained images of the past - things that had been seen in them. Maybe the Stone of Erech would have shown the original meeting of Isildur & the men of the Mountain, kind of duplicating the moment of their swearing of service to Isildur & his heir - pure speculation for novelty value.

I don't think you need to speculate. I noticed Gandalf's words in the final chapter of Book 3:

And how it draws one to itself! Have I not felt it? Even now my heart desires to test my will upon it, to see if I could not wrench it from him and turn it where I would--to look across the wide seas of water and of time to Tirion the Fair, and perceive the unimaginable hand and mind of Feanor at their work, while both the White Tree and the Golden were in flower!" He sighed and fell silent.

Gandalf, in wistful fashion, is relating yet another secret which the palantiri hold, that they retain visions of the past.