What did they see, what could they see at a distance so uncertain that Barbican has never been able even to guess at it?
Not much.

Ardan was reminded of the night he had stood on the battlements of Dover Castle, a few years before, when the fitful flashes of a thunder storm gave him occasional and very uncertain glimpses of the French coast at the opposite side of the strait.

Misty strips long and narrow, extending over one portion of the disc--probably cloud-scuds sustained by a highly rarefied atmosphere--permitted only a very dreamy idea of lofty mountains stretching beneath them in shapeless proportions, of smaller reliefs, circuses, yawning craters, and the other capricious, sponge-like formations so common on the visible side.

Elsewhere the watchers became aware for an instant of immense spaces, certainly not arid plains, but seas, real oceans, vast and calm, reflecting from their placid depths the dazzling fireworks of the weird and wildly flashing meteors.
Farther on, but very darkly as if behind a screen, shadowy continents revealed themselves, their surfaces flecked with black cloudy masses, probably great forests, with here and there a-- Nothing more! In less than a second the illumination had come to an end, involving everything in the Moon's direction once more in pitchy darkness.
But had the impression made on the travellers' eyes been a mere vision or the result of a reality?
an optical delusion or the shadow of a solid fact?
Could an observation so rapid, so fleeting, so superficial, be really regarded as a genuine scientific affirmation?
Could such a feeble glimmer of the invisible disc justify them in pronouncing a decided opinion on the inhabitability of the Moon?
To such questions as these, rising spontaneously and simultaneously in the minds of our travellers, they could not reply at the moment; they could not reply to them long afterwards; even to this day they can give them no satisfactory answer.
All they could do at the moment, they did.

To every sight and sound they kept their eyes and ears open, and, by observing the most perfect silence, they sought to render their impressions too vivid to admit of deception.
There was now, however, nothing to be heard, and very little more to be seen.