While the "Mona Lisa" was undoubtedly the greatest of Leonardo's portraits, perhaps the best possible idea of Leonardo's power as an artist is to be found in the "Last Supper." Instead of making a placid group he has chosen the moment just after the Lord had said that one of the Twelve will betray Him and when all are asking "Is it I, Lord?" He represents not only the individual shock but also the natural grouping that occurred as a consequence of the announcement There are four groups of three each, separate and with very distinct interest and yet they are so arranged as not to break the unity of the picture. On the left side the outer three { 17} are all intently gazing on the Lord while the external group on the other side are gazing away from Him, but their hands all point towards Him. The inner three on the right are talking directly to Him while the corresponding three on the left are occupied among themselves and yet evidently intent on Him. There was probably never put together a more expressive set of faces. Each one is eminently individual, and each one shows marvellously the character of the Apostle represented. It has been said that it is as if the painter had made a condensed biography of each one of them with his brush. All the special characteristics of the different Apostles that we know are here to be seen in their faces. He has painted the souls and characters of the men in the imaginary portraits that he makes.
There is an old tradition mentioned by Vasari, that charming gatherer of legends with regard to the old painters, that Leonardo, unable to satisfy himself with the head and face of Jesus, left it unfinished. This would indeed have been a sad loss to art. Leonardo hesitated for long, wondering above all whether he should follow a model, but finally made his peace with tradition, accepted the type of head for the Lord that had been created by Giotto, and refining it still more succeeded in giving a look of mystic superhumanity to it that would evoke the idea of divinity. It is easy to see how much he borrowed but it is harder to realize how much he added, yet artists who have studied it have felt that here indeed was a triumph and that, as far as possible, Leonardo had represented the human face divine. He followed his model strictly in the case of the Apostles' heads and none knew better than he how to select models, but in the head of Christ he turns from the model and works out his design from ideals of the human face of which so many existed in his well-stored fancy. The face of Christ was left somewhat vague, trembling, undissolved like faces seen in cloud or in the fire. Leonardo himself once counselled his students to look for suggestions in curious cloud and fire shapes and even to study the vague forms that occur in imitation of human faces on cracked and stained surfaces of ruined walls, and some of his own devotion to this seems to have been of help to him in this marvellous face.