365. _The first condition, which must be fulfilled by every work of art, is, that it be presented without confusion and as distinctly as possible._ Let us add that paintings on glass, executed on the method of _chiaro-’scuro_, cannot receive the borders and grounds of rose windows which have such fine effects of colour, as they have less brilliancy and transparency than the glass in which the colouring material has been incorporated; they are also less capable of resisting the injuries of time. Variety of colours in these windows is so necessary to attain the best possible effect, that those which represent figures entirely nude, edifices, or large objects of a single colour, or slightly tinted, whatever may be the perfection of their execution with regard to finish or truth of imitation, will have an inferior effect to windows composed of pieces of varied colours suitably contrasted; but a bad effect results from the mixture of coloured glass with transparent colourless glass, when the latter has a certain extent of surface in a window; yet a good effect is obtainable by mixing ground glass with coloured glass, and also of small pieces of colourless transparent glass, framed in lead, so that at the distance at which they must be viewed they produce the effect of a symmetrical juxtaposition of white parts with black parts. 366. I conclude that we must refer the causes of the beautiful effects of coloured glass of great churches— 1. To their presenting a very simple design, whose different well-defined parts may be seen without confusion at a great distance. 2. To their offering a union of coloured parts which are distributed with a kind of symmetry, but which are also vividly contrasted, not only among themselves, but also with the opaque parts which surround them. 367. Coloured windows appear to me to produce their utmost effect only in the vast edifices where the different-coloured rays reach the eye of the spectator on the floor of the church so much scattered that they impinge upon each other, whence results an harmonious mixture, not found in a small structure lighted by coloured windows. It is this intimate mixture of the coloured rays, transmitted into a vast edifice, which permits of tapestries placed on the ground floor. But when the lower walls have not colourless glass windows, it is evident that, if tapestries be placed too near coloured windows, the harmony of their colours must be lost, as when blue rays fall upon red draperies, yellow rays upon blue draperies, &c. Thus, when coloured glass is to be put in a window, it is necessary to take into consideration, not only its beauty, but also the effect which the coloured light it transmits will have upon the objects illuminated by it.