What the masters of Barbizon had done through instinctive genius was made the object of scientific study by the Impressionists. The new school set up the principle that atmosphere changes the colour of objects; for instance, that the colour and outline of a tree painted in a room are completely different from those of the same tree painted upon the spot in the open air. As an unqualified rule they claimed that every incident was to be harmonised with time, place, and light; thus a scene taking place out of doors had of necessity to be painted, not within four walls, but under the actual illumination of morning, or noon, or evening, or night. In making this problem the object of detailed and careful inquiry the artist came to analyse life, throbbing beneath its veil of air and light, with more refinement and thoroughness than the old masters had done. The latter painted light deadened in its fall, not shining. Oils were treated as an opaque material, colour appeared to be a substance, and the radiance of tinted light was lost through this material heaviness. Courbet still represented merely the object apart from its environment; he saw things in a plastic way, and not as they were, bathed in the atmosphere; his men and women lived in oil, in brown sauce, and not where it was only possible for them to live--in the air. Everything he painted he isolated without a thought of atmospheric surroundings. Now a complete change of parts was effected: bodies and colours were no longer painted, but the shifting power of light under which everything changes form and colour at every moment of the day. The elder painters in essentials confined light to the surface of objects; the new painters believed in its universality, beholding in it the father of all life and of the manifold nature of the visible world, and therefore of colour also. They no longer painted colours and forms with lights and cast-shadows, but pellucid light, pouring over forms and colours and absorbed and refracted by them. They no longer looked merely to the particular, but to the whole, no longer saw nothing except deadened light and cast-shadows, but the harmony and pictorial charm of a moment of nature considered as such. With a zeal which at times seemed almost paradoxical, they proceeded to establish the importance of the phenomena of light. They discovered that, so far from being gilded, objects are silvered by sunlight, and they made every effort to analyse the multiplicity of these fine gradations down to their most delicate _nuances_. They learnt to paint the quiver of tremulous sunbeams radiating far and wide; they were the lyrical poets of light, which they often glorified at the expense of what it envelops and causes to live. At the service of art they placed a renovated treasury of refined, purified, and pictorial phases of expression, in which the history of art records an increase in the human eye of the sense of colour and the power of perception.