One of the 29 bronze castings of Degas's most famous sculpture, La Petite Danseuse de Quatorze Ans, has been lent by the St Louis Art Museum to the National Gallery of Australia, to serve as the centrepiece of its current exhibition, Degas: Master of French Art. In another room stands a nude version of the same figure, modelled some time between 1878 and 1881. The subject was a 14-year-old dancer called Marie de Goethem, who posed for Degas several times, nude and in her tutu. The subject was popular; to the 29 castings of the clad version made after the artist's death must be added 26 castings of the nude version. Recent research is held to have proved conclusively that the nude figure is not a preparatory study for the clothed one, but an individual work, of which Degas had a casting made in bronze.

The first exhibition of the clad figure caused an outcry, not because the subject was a child, but because she was so unattractive. Jules Claretie reported in Le Temps that he found the wax figure "peculiarly disturbing". "The lecherous little snout on this barely pubescent young girl, this little flower of the gutter, is unforgettable," he wrote. Paul Mantz's response was similar: "With bestial effrontery she moves her face forward, or rather her little muzzle - and this word is completely correct because the little girl is the beginning of a rat." (The adolescent corps de ballet at the Paris Opera were known as petits rats.) He goes on:

"Why is she so ugly? Why is her forehead, half covered by her hair, marked already, like her lips, with a profoundly vicious character?"

Today's public is not likely to make a lecherous interest in the body of a child the fault of the child herself, if only because so few of us are the kind of bowler-hatted, cigar-smoking, clubbable gentlemen who prowled the backstage corridors of the Paris Opera in the 1880s. If the nude figure is disturbing, it is because the child is underdeveloped for her stated age, because her breasts are mere buds on her narrow ribcage, because her pelvis is shallow and unformed and her belly slack and protuberant, because her thighs are wasted and her knees almost rachitic. This is what passed by gas footlights for a sylph - an undernourished child for whom dancing was a one-way ticket to prostitution.

Degas is certainly responsible for stripping the figure of the adolescent dancer of cuteness, and we could argue that his intent is partly moral, but anyone who looks for compassion in the work will not find it. The visual language of compassion was unusable for any serious artist in the 1870s and 80s, because the public art of the period oozed sentiment. Pretty beggars and plump rosy little girlies with tears in their eyes were as often to be encountered then, as fluffy kittens are today. Degas dispensed with pathos as summarily as he dispensed with glazes. His surfaces get thinner and poorer, the pigment goes on drier and drier. Line becomes more and more important, burning through the bursts of aniline colour that begin to dominate in the later work, like the skull beneath the stage-lit skin.

The figure of the sexual predator is a constant in Degas's image-making. Portly gentlemen in solid black are forever watching the girls, in their dance classes, in the dressing rooms, in the wings, from the stalls. A monotype in the possession of the NGA shows a group of four top-hatted gentlemen towering over two tiny dancers they have cornered in a backstage corridor. Degas captures the cocky bravado of the little girls, who are looking to make the best of an inescapable situation.

When the work comes together, as it has in Canberra, it becomes apparent that Degas's detachment is beyond cruelty. He is like the painter in Zola's novel, L'Oeuvre, who cannot stop painting his wife's dead face because he is fascinated by the way the colour of her skin is changing. How you react to what Degas shows you is none of the artist's concern. There can be as little doubt that Degas used prostitutes as that he used laundresses and ironing ladies. He was aware of women as independent beings, and had more respect for women artists - for Cassatt, Morisot, and Valadon, for example - than any of his contemporaries, but they were not his subject. His subject, when it is not horses, is the interaction of gentlemen and labouring women, whether dancers, prostitutes or laundresses. Sex is always part of the relationship, but love has nothing to do with it.

Degas's brother René is said to have destroyed 70 pornographic sketches that were found at the time of the artist's death. One escaped and can be seen in this exhibition. Joris-Karl Huysmans was troubled by what he saw as "scorn and loathing" for women in Degas's work. It is undeniable that most of Degas's women are faceless and abject, foreshortened heaps of limbs and buttocks, but ultimately it is less insulting to women to show their bodies coarsened by privation and hard work, by age and ill health, than it is to show them forever delectable and young.