The Impressionists were not the first artists to work out-of-doors: even teachers at the stuffy French academy encouraged students to make painting expeditions to the countryside. But most artists simply made rough sketches in the open air; they made full works on canvas only in the studio. The spontaneity required to complete a painting in one outing is what distinguished the Impressionists' approach. Plein-air painting became more practical in the mid-19th century with the invention of metal paint tubes and portable easels. Painting in all kinds of weather, Van Gogh ingeniously adapted his equipment to whatever conditions he encountered. On windy days, he secured his easel to the ground with ropes, and at night, he sometimes painted by candlelight.