Georg inverted six of the larger plates so that I could see them. ‘You recognize these, of course?’

I nodded. With the exception of one, Rubens’ Pieta in the Hermitage Museum at Leningrad, I had seen the originals of them all within the previous five years. The others were the missing Leonardo Crucifixion, the Crucifixions by Veronese, Goya and Holbein, and that by Poussin, entitled The Place of Golgotha. All were in public museums the Louvre, San Stefano in Venice, the Prado and the Ryksmuseum, Amsterdam — and all were familiar, wellauthenticated masterworks, centrepieces, apart from the Poussin, of major national collections. ‘It’s reassuring to see them. I trust they’re all in good hands. Or are they next on the mysterious thief’s shopping list?’

Georg shook his head. ‘No, I don’t think he’s very interested in these. Though he keeps a watching brief over them.’ Again I noticed the marked change in Georg’s manner, the reflective private humour. ‘Do you notice anything else?’

I compared the photographs again. ‘They’re all crucifixions. Authentic, except perhaps in minor details. They were all easel paintings.’ I shrugged.

‘They all, at some time, have been stolen.’ Georg moved quickly from right to left. ‘The Poussin from the Chateau Loire collection in 1822, the Goya in 1806 from the Monte Cassino monastery, by Napoleon, the Veronese from the Prado in 1891, the Leonardo four months ago as we know, and the Holbein in 1943, looted for the Herman Goering collection.’

‘Interesting,’ I commented. ‘But few masterworks haven’t been stolen at some time. I hope this isn’t a key point in your theory.’

‘No, but in conjunction with another factor it gains insignificance. Now.’ He handed the Leonardo reproduction to me. ‘Anything unusual there?’ When I shook my head at the familiar image he picked up another photograph of the missing painting. ‘What about that one?’

The photographs had been taken from slightly different perspectives, but otherwise seemed identical. ‘They are both of the original Crucifixion,’ Georg explained, ‘taken in the Louvre within a month of its disappearance.’

‘I give up,’ I admitted. ‘They seem the same. No — wait a minute!’ I pulled the table light nearer and bent over the plates, as Georg nodded. ‘They’re slightly different. What is going on?’

Quickly, figure by figure, I compared the photographs, within a few moments seized on the minute disparity. In almost every particular the pictures were identical, but one figure out of the score or more on the crowded field had been altered. On the left, where the procession wound its way up the hillside towards the three crosses, the face of one of the bystanders had been completely repainted. Although, in the centre of the painting, the Christ hung from the cross some hours after the crucifixion, by a sort of spatio-temporal perspective — a common device in all Renaissance painting for overcoming the static nature of the single canvas — the receding procession carried the action backwards through time, so that one followed the invisible presence of the Christ on his painful last ascent of Golgotha.

The figure whose face had been repainted formed part of the crowd on the lower slopes. A tall powerfully built man in a black robe, he had obviously been the subject of special care by Leonardo, who had invested him with the magnificent physique and serpentine grace usually reserved for his depiction of angels. Looking at the photograph in my left hand, the original unretouched version, I realized that Leonardo had indeed intended the figure to represent an angel of death, or rather, one of those agents of the unconscious, terrifying in their enigmatic calm, in their brooding ambivalence, who seem to preside in his paintings over all man’s deepest fears and longings, like the grey-faced statues that stare down from the midnight cornices of the necropolis at Pompeii.

All this, so typical of Leonardo and his curious vision, seemed to be summed up by the face of this tall angelic figure. Turned almost in profile over the left shoulder, the face looked up towards the cross, a faint flicker of pity investing the grey saturnine features. A high forehead, slightly flared at the temples, rose above the handsome semitic nose and mouth. A trace of a smile, of compassionate resignation and understanding, hung about the lips, providing a solitary source of light which illuminated the remainder of the face partly obscured by the shadows of the thundering sky.

In the photograph on my right, however, all this had been altered completely. The whole character of this angelic figure had been replaced by a new conception. The superficial likeness remained, but the face had lost its expression of tragic compassion. The later artist had reversed its posture altogether, and the head was turned away from the cross and over the right shoulder towards the earthly city of Jerusalem whose spectral towers rose like a city of Miltonic hell in the blue dusk. While the other bystanders followed the ascending Christ as if helpless to assist him, the expression on the face of the black-robed figure was arrogant and critical, the tension of the averted neck muscles indicating that he had swung his head away almost in disgust from the spectacle before him.

‘What is this?’ I asked, pointing to the latter photograph. ‘Some lost pupil’s copy? I can’t see why—’

Georg leaned forward and tapped the print. ‘That is the original Leonardo. Don’t you understand, Charles? The version on your left which you were admiring for so many minutes was superimposed by some unknown retoucher, only a few years after da Vinci’s death.’ He smiled at my scepticism. ‘Believe me, it’s true. The figure concerned is only a minor part of the composition, no one had seriously examined it before, as the rest of the painting is without doubt original. These additions were discovered five months ago shortly after the painting was removed for cleaning. The infra-red examination revealed the completely intact profile below.’

He passed two more photographs to me, both large-scale details of the head, in which the contrasts of characterization were even more obvious. ‘As you can see from the brush-work in the shading, the retouching was done by a right-handed artist, whereas we know, of course, that da Vinci was left-handed.’

‘Well…’ I shrugged. ‘It seems strange. But if what you say is correct, why on earth was such a small detail altered? The whole conception of the character is different.’

‘An interesting question,’ Georg said ambiguously. ‘Incidentally, the figure is that of Ahasuerus, the Wandering Jew.’ He pointed to the man’s feet. ‘He’s always conventionally represented by the crossed sandal-straps of the Essene Sect, to which Jesus himself may have belonged.’

I picked up the photographs again. ‘The Wandering Jew,’ I repeated softly. ‘How curious. The man who taunted Christ to move faster and was condemned to rove the surface of the earth until the Second Coming. It’s almost as if the retoucher were an apologist for him, superimposing this expression of tragic pity over Leonardo’s representation. There’s an idea for you, Georg. You know how courtiers and wealthy merchants who gathered at painters’ studios were informally incorporated into their paintings — perhaps Ahasuerus would move around, posing as himself, driven by a sort of guilt compulsion, then later steal the paintings and revise them. Now there is a theory.’

I looked across at Georg, waiting for him to reply. He was nodding slowly, eyes watching mine in unspoken agreement, all trace of humour absent. ‘Georg!’ I exclaimed. ‘Are you serious? Do you mean—’

He interrupted me gently but forcefully. ‘Charles, just give me a few more minutes to explain. I warned you that my theory was fantastic.’ Before I could protest he passed me another photograph. ‘The Veronese Crucifixion. See anyone you recognize? On the bottom left.’