The static elements of the glass and its architectural setting are modified by the element of change inherent in natural light. A seemingly endless spectrum of changes in the appearance of stained glass is a result of the changes in the intensity, disposition, atmospheric diffusion, and colour of natural daylight. The luminous life of stained glass, therefore, can best be observed by watching the organic effect of light on the window through the course of a day. If one were to enter Chartres Cathedral just after sunrise on the morning of a clear day, it would be to the east windows, especially those in the clerestory, that one’s eyes would first be drawn. They alone will have come fully to life, and all of the others will still seem to half-exist in a kind of hushed twilight. Gradually, as the sun rises in the sky, these windows will become more luminous. Then the east windows will begin to lose their earlier brilliance to those all along the south flank of the cathedral, which by midday will be fairly aglow from the direct rays of the sun. The light streaming through the south windows, however, will have raised the light level inside the north windows opposite them sufficiently to create a distinct, though by no means unpleasant, muting of the radiance of the latter. If the sun at this point disappears behind a cloud and the sky becomes generally overcast, the appearance of all of the windows is immediately and dramatically altered. Because the light, now diffused, comes more or less equally from all directions, the south windows will lose some of their earlier brilliance and vivacity and the north windows will recover theirs. The overall atmosphere of the cathedral is distinctly cooler and graver in its effect, and more than ever before one begins to become aware of absolute differences in the tonality of the various windows themselves. The grisaille windows in the east end of the cathedral, the highly keyed 15th-century window in the Vendôme Chapel in the south aisle of the nave, and the three 12th-century windows over the great west portal all stand out as being substantially more luminous than the rest. If, late in the afternoon, the sun reappears, the viewer is treated to an extraordinary spectacle as the blues in the west windows, by far the most intense in the cathedral, are further emblazoned by the direct rays of the sun. Should the main doors of the cathedral be opened, the direct rays of the late afternoon sun, streaming halfway down the nave of the cathedral, will cast a blinding pall over all the windows within their vicinity until the doors are closed once more. Then as the sky begins to redden with the setting sun, the intense 12th-century blues in the west windows lose their former intensity, and the warmer colours, especially the rubies, become so fiery and assertive that they seem almost to have displaced the blues as the predominant colour in the windows. Finally, when the sun is gone the whole cathedral is plunged once more into a deep twilight, which gradually diminishes until there is no light at all.