Mieris the Elder was initially trained as a goldsmith, and probably turned to painting around 1651. He is considered one of the most important Dutch fijnschilders, whose work was characterised by fine and precise brushwork. In spite of the fine detail, it's not easy to say where the thumb actually IS -- it looks like it's resting on the side of the frog - or perhaps under at the very back. In any case, although the high third and fourth fingers are typical for the time, the entire hand position is impossibly awkward in practice, with the bow stick touching the hand between the thumb and forefinger.