Courbet's art, like his life, did decline in exile. One of the best pictures here, ''Fox in the Snow,'' is from around 1860, hardly late in Courbet's life. It shows a landscape built up by layers of pigment as solid and packed as the snow. As often with him, there is the evocation of muffled air: the air as something tangible and unchanging, like stone. This is not pretty painting. It's a brutish picture, a wounded animal starkly silhouetted, painted with a sensuous concentration that seems almost akin to love.