Two years ago, the director of the National Gallery, Neil MacGregor, made a fine series for BBC2 called Painting the World. Despite a plummy accent, the appearance of a chartered accountant and a liking for long words, he turned out to be a very effective presenter. He not only knew what he was talking about, but clearly felt that it mattered. He now continues his television career with what, for anyone studying art or art history at almost any level, could be the most revealing and useful three hours ever broadcast.
He starts by going round the back of the painting - in his first example, a Rubens portrait which has been painted on three panels of wood, plus two added strips: that explains a slight variation in colour on the painted side. The grey of the sky in Veronese's "Vision of St Helena" is more simply explained: the blue pigment known as smalt has faded, and only a computer simulation can give an impression of what the painter originally intended. A colour change in Renoir's "The Umbrellas" has a more complicated history, involving a visit to Cezanne and a development in women's fashions. One theme of the series turns out to be the importance of understanding how paintings alter with time.
However, the dominant topic from programme two onwards is the difference between egg tempera and oil paint. Upstairs, in the gallery's conservation department, a restorer demonstrates how a wood panel would have been prepared with canvas and gypsum, then painted with tempera and gold leaf, to make a 14th-century Florentine altarpiece. The same basic technique was used in England for the "Wilton Diptych", a painting that illustrates both the beauty and the limitations of egg yolk as a medium. Something surprising happens when the Diptych is lit, as it would originally have been, by candlelight.
The introduction of oil paint brought a revolution in the way that light could be depicted, as MacGregor demonstrates in the third programme by a comparison between the representation of Christ's birth on the Florentine altarpiece and the same scene in a contemporary painting from the Netherlands. He develops the idea in a close examination of the "Arnolfini Wedding", by Jan van Eyck, and the following week in paintings by Caravaggio, Van Dyck and Rubens.
After that, he shows the changes made possible by the introduction of machine-made pigments, which were to allow Van Gogh to live in the south of France and order a huge range of different shades from his colourman in Paris. As well as showing the artist's inner turmoil, MacGregor suggests, Van Gogh's paintings are evidence of a "flourishing paint industry and an efficient post service".
Finally, he turns to the subject of conservation, a concern of the National Gallery since the discovery, in the 19th century, that London smog was not an ideal medium in which to keep paintings. Ill-advised alterations, poor storage and even bayonetting by French soldiers (all before the works in question came into the care of the National Gallery) present their own problems. MacGregor skirts around the controversial aspects of restoration, emphasising what it can teach about an artist's methods and a painting's history. This is a series about demonstrable facts, not dubious judgments, and it leaves you with the feeling of suddenly seeing and understanding more clearly.