The colouration of the windows had the advantage of throwing on the opaque walls a veil, or coloured glazing, of extreme delicacy, always assuming that the coloured windows themselves were harmoniously toned. Whether their resources did not permit the artists to adopt a complete system of coloured glass, or whether they wanted to get daylight in purer quality into their interiors,-- whatever may have been their reasons,--they resorted to this beautiful grisaille decoration which is also a colouring harmony obtained by the aid of a long experience in the effects of light on translucent surfaces. Many of our churches retain grisaille windows filling either all, or only a part, of their bays. In the latter case, the grisailles are reserved for the side windows which are meant to be seen obliquely, and in that case the coloured glass fills the bays of the fond, the apsidal openings which are meant to be seen in face from a distance. These lateral grisailles are still opaque enough to prevent the solar rays which pass through them from lighting the coloured windows on the reverse side; yet, at certain hours of the day, these solar rays throw a pearly light on the coloured windows which gives them indescribable transparence and refinement of tones. The lateral windows in the choir of the Auxerre Cathedral, half-grisaille, half-coloured, throw on the wholly coloured apsidal window, by this means, a glazing the softness of which one can hardly conceive. The opaline light which comes through these lateral bays, and makes a sort of veil, transparent in the extreme, under the lofty vaulting, is crossed by the brilliant tones of the windows behind, which give the play of precious stones. The solid outlines then seem to waver like objects seen through a sheet of clear water. Distances change their values, and take depths in which the eye gets lost. With every hour of the day these effects are altered, and always with new harmonies which one never tires of trying to understand; but the deeper one's study goes, the more astounded one becomes before the experience acquired by these artists, whose theories on the effects of colour, assuming that they had any, are unknown to us and whom the most kindly-disposed among us treat as simple children.