THERE can't be much doubt about what it was in Holbein the Younger's portraits that appealed to his contemporaries: their realism. Erasmus, when he saw Holbein's group portrait of Sir Thomas More and family, wrote to Sir Thomas in England that it would be impossible for him to see the family more clearly if he were there.
In the portrait of the Hanseatic merchant George Gisze, a note is attached to the green timber wall of his office, just above his head. According to one translation this reads:
''This picture you see shows George as he really looks, his features, the life in his eyes, the very shape of the man.''
And, as court painter for Henry VIII, Holbein had the extraordinary ability to depict the physical appearance of his subjects with clear, undeniable authority and realism that most impressed people in his time. One of his portraits of the king himself was in fresco on a wall at Whitehall, and visitors are supposed to have believed they were actually being ushered into the royal presence when they first caught sight of it. As late as 1604, Van Mander described himself as feeling ''abashed or annihilated before (this) figure of the king.'' (We have no way of judging today, as the fresco was destroyed later by fire.)
So Holbein's essential value to his clientele was his skill in producing what John Pope-Hennessy (in his book ''The Portrait in the Renaissance'') has called the ''substitute life image.'' The portrait was a stand-in for the person portrayed. Henry even sent Holbein to paint his prospective brides, so that he could size them up before meeting them.
But ''realism'' is an odd commodity. It can contain a considerable degree of artifice. A scrupulous accuracy, or ''likeness,'' is certainly an aspect of Holbein's portraiture, and it has even been hazarded that in his later English portraits the German-born painter might have employed a tracing device when drawing the features of his distinguished subjects. But it is evident that he was not simply or solely aiming at correctness and recognizability. Status, success, and importance also motivate these paintings of men and women, and in the case of the king, the propagandist element of power is a definite ingredient.
Drawing - very fine drawing - is the basis of his painting style: eyes, noses , lips, the definitions of cheek and brow, even the individual hairs of beards or on heads, of eyebrows and lashes, are shown in Holbein's suberb, immaculate language of line. But this line is not so much fluid or momentary as describing a kind of permanence. The faces of his sitters seem intent and unmoving - not lacking in vitality - but set, as though, once he had caught their essence, they would never again change. Pope-Hennessy discusses Holbein's aim to ''establish'' a face and a pose, to arrive at a ''considered statement on the personality,'' which could even be used repetitively when further portraits of an individual were required from the artist's workshop.
Not without significance in this respect is the fact that Holbein had been a dedicated religious painter before his move to England in 1532, and was therefore practiced in the depiction of human beings as emblematic, as figures symbolic of certain attitudes and motives. Reform in the Roman Catholic church had, however, threatened his very livelihood, and his move from Basle (now Basel) - where iconoclasm and dreadful violence had come to a pitch - to England (which was soon to undergo similar destructiveness) was paralleled by his artistic move from a painter of sacred to a painter of secular images. At this period people were neither clearly ''protestant'' nor ''catholic''; lines were fuzzed. But the painted images in churches were under ferocious attack, and, whatever his sympathies, Holbein must have felt impelled to pursue a different branch of the painter's craft. Fortunately England gave him plenty of opportunity.
So his subject matter changed, but his characteristic approach to the human psyche remained, in certain penetratingly objective ways, the same. Instead of the ''nativity,'' in which the infant Jesus and his mother are surrounded by gifts and offerings, we have a merchant surrounded by his possessions. (German merchants in England were among Holbein's more frequent customers before he was taken on by the court.) Instead of the rich stuff of the Virgin's robe or the opulent costumes of the three kings, we have the finery of George Gisze. Instead of the flowers sometimes placed on the ground in Flemish paintings of the Nativity, we have an elegant Murano glass vase containing delicately painted pinks - symbolizing George's engagement to be married.
Holbein has transferred these exquisite still-life details, all contributing to the splendor and industry of Gisze's self-image, from church altar to countinghouse. But they still carry the significance of uniquely valued objects depicted with meticulous attention and skilled pleasure. They support the emblem - the use of painting to praise an individual or event as above the ordinary.
The amazing thing about Holbein's emblematic portraits, though, is that he simultaneously manages such a knowing appraisal of the sitter's character that you feel you've met him or her face to face. There is an extraordinary frankness , a lack of flattery, in these balanced, tranquil portraits. The faces seem both inevitable and unique. The ''life in his eyes, the very shape of the man,'' indeed: and although George Gisze's features are handsome enough, one feels sure that Holbein would not have shrunk from recording any less usual or felicitous traits the merchant might have had. And when you look at his later pictures of the great and noble (and sometimes the canny and unscrupulous) men and women of Henry's court, this is precisely the case. ''We do not doubt for a moment that they are in fact faithful records of what Holbein saw, drawn without fear or favor,'' is what Ernst Gombrich wrote of them.