Overhead was a complex assemblage of apparatus: bulbous, silvered tubes, small electric motors that hummed in various unexpected places, makeshift screens of zinc, roughly soldered, coils upon coils of wire, and a network of slung cables that made the place look like a creeper-tangled tropical jungle. A large dynamo churned out a steady roar in the corner, and a pair of wide sparkgaps crackled continuously, filling the laboratory with a weird, jumping blue light as the day waned outside the windows and the dusk crept in.

(Duplication chamber from 'The 4-Sided Triangle' by William F. Temple)

An intruder in the laboratory might have looked through the window of the other container and seen, standing on a steel frame in a cubical chamber, an oil painting of “Madame Croignette” by Boucher, delicately illuminated by concealed lights. He would not have known it, but the painting was standing in a vacuum.

If he had squeezed behind the trio at the other container and gazed through their window he would have seen an apparently identical sight: an oil painting of “Madame Croignette” by Boucher, standing on a steel frame in a vacuum, delicately illuminated by concealed lights.

From which he would probably not gather much.

The catch was that the painting at which the three were gazing so intently was not quite the same as the one in the first container — not yet. There were minute differences in color and proportion.

But gradually these differences were righting themselves, for the whole of the second canvas was being built up atom by atom, molecule by molecule, into an exactly identical twin of the one which had felt the brush of Francois Boucher.