John Ruskin was on the edge of mental collapse. You can see it coming in his fearsome, almost impossible-to-look-in-the-eye Self-Portrait, a watercolour of about 1875, in which his brow, like some monstrous geological formation, presses down on his shadowed left eye and his face seems about to twist out of shape under the tension. The sky behind him is yellow and sick. The Guardian: Artists v critics, round one