Degas is quoted as saying, ''There is nothing less spontaneous than my art.'' And yet his paintings consistently have the air of spontaneity. He is the master of the captured moment, and of what looks like a fragment, arbitrarily noticed: nothing but the legs of a dancer, all but hidden from view by each other, visible under a falling stage curtain. A milliner sliced in half by a column, the hat she offers a customer seen but not the arm that passes it: just how much calculation was probably Degas' secret.
He practised with conscious ingenuity all the wiles of an art that hides art. He had an uncanny sense of interval, and sometimes seems to be testing the extent to which figures can move in opposition to each other, or can open up an unusually wide space between them. At other times, as in ''A la Bourse,'' he crowds figures almost (but not quite) to the point of visual confusion. It is as if he is asking how casually true to life a painting can be, and still have sufficient balance and construction.
He studied the movement of dancers, surely not only as paintable modern subject matter but as the practise of an art form that offered a stimulating parallel to his own practise of painting and drawing.
Paintings are, of course, stationary. It is this fact that gives a special fascination to the question of movement expressed by them. Music has its own time scale. Dance translates time into the language of space. Painting can really only operate in space, so that it has to render movement by means of its singular immediacy - a significant problem for almost all painting.
If one were to study the history of painting from this angle, Degas' art would rate as the most original of investigations. Virtually everything he painted or drew in his maturity is in movement, not just in very evident actions , but telling gestures, changing facial expressions, even states of equilibrium or balance that can only be maintained for an instant.
The dealer Ambroise Vollard tells a story about Degas which indicates how vital the notion of changing movement was to his art. Vollard suggested having one of his wax figures of dancers cast.
''Have it cast!'' Degas cried. ''Bronze is all right for those who work for eternity. My pleasure consists in beginning over and over again. Like this . . . look.''
Vollard says he then took an almost finished danseusem from his modelling stand and rolled her into a ball of clay.
Perhaps Degas was working in an area of paradox and knew it. He wanted his art to be both spontaneous and scrupulous, both planned and unexpected.
Particularly interesting in these respects are his portraits, because portraiture, by its traditional nature, demands stillness. It is a study of inner, rather than outer, life, of motivation rather than motion. It cannot, therefore, if it is true portraiture, be too cold or detached, and detachment is one of the things Degas is sometimes criticised for. He is accused of so much interest in the actions of his dancers, laundresses and jockeys that he ignores their inner life. I believe this to be a misjudgment, or at least a wrong emphasis. It is revealing, for instance, that he depicts these people before and after, quite as much as during, the performance of their art, sport or job. He delights in the behind-the-scenes view of things, the actual behind the artificial - dancers rehearsing, jockeys warming up before the race, laundresses pausing in their work and yawning or reading a letter. There is often an undeniable intimacy in his observations.
There are many subtle indications of his understanding of and sympathy for the people he paints. Again, it is art hiding art. His reputation for cool objectivity is really a myth which the essential privacy of his art denies. He is not extrovert in his expression of feeling and emotion - but that is quite different from such expression's not being there at all.
''A la Bourse'' (''At the Stock Exchange'') is a lightly misleading title for this painting from the Louvre, because it is in fact a portrait. It might indeed be taken at first or second glance to be Degas bringing his study of gesture and movement to bear on the business world rather than the dance world.
The central figure however, is Ernest May, a Jewish banker and collector of Impressionist paintings.
What an original approach to portraiture it is. Instead of a static ''sitter, '' Degas shows May at work, natural, absorbed, unaware of the artist. There is no self-conscious pose, no special-occasion vanity, no artificial setting. Once again it is a picture full of movement, a fragment of life. It is like an instant photograph, not only in its apparently casual glimpse of top-hatted financiers who happen to have fallen into this momentary kinetic pattern, but also in its cunning use of photographic blur and fuzz. Everything except May is more or less out of focus: he is the still point at the centre of bustle and activity, and we see only his features with clarity - his sensitive intelligence , his shrewdness, his self-control,
Degas demonstrates in ''A la Bourse'' that it is possible to portray both the outer and the inner life of a man in terms of his activity. He does not have to be a frozen statue. It is as though movement itself is, in Degas's eyes, so intrinsic to the life of a man that to portray him in a state of inactivity would not be realistic or true.