En plein air (; French for 'outdoors'), or plein air painting, is the act of painting outdoors. This method contrasts with studio painting or academic rules that might create a predetermined look. The theory of 'En plein air' painting is credited to Pierre-Henri de Valenciennes (1750–1819), first expounded in a treatise entitled Reflections and Advice to a Student on Painting, Particularly on Landscape (1800), where he developed the concept of landscape portraiture by which the artist paints directly onto canvas in situ within the landscape. It enabled the artist to better capture the changing details of weather and light. The invention of portable canvases and easels allowed the practice to develop, particularly in France, and in the early 1830s the Barbizon school of painting in natural light was highly influential. Amongst the most prominent features of this school were its tonal qualities, colour, loose brushwork, and softness of form. These were variants that were particularly relevant to the mid 19th-century Hudson River School and to Impressionism. History Before the 19th century, artists had mixed their own paints from raw pigments that they often ground themselves from a variety of media. This had made for inconvenient portability and kept most painting activities confined to the studio. This changed in the 1800s when tubes of oil paint became available, allowing En plein air painting to become viable for many artists. In the 1830s, the Barbizon school in France that included Charles-François Daubigny and Théodore Rousseau used the practice to accurately depict the changing appearance of light as weather conditions altered. In the early 1860s, four young painters: Claude Monet, Pierre-Auguste Renoir, Alfred Sisley and Frédéric Bazille, met whilst studying under the academic artist Charles Gleyre. They discovered that they shared an interest in painting landscape and contemporary life, and they often ventured into the countryside together to paint in the open air. They discovered that they could paint in sunlight directly from nature, and making use of the vivid synthetic pigments that were available, they began to develop a lighter and brighter manner of painting that extended further the Realism of Gustave Courbet and the Barbizon school. It was radical practise at its inception, but by the later decades of the 19th-century the theory had been absorbed into normal artistic practise. There were artists' colonies across France, such as the one at Étaples on the Côte d'Opal that included landscape impressionists Eugène