Conformably to the principle enunciated above (346), of judging the productions of art by the rules drawn from the nature of the materials employed, I establish two distinct classes of churches, not according to their form, but to a fundamental consideration which subordinates the interior decoration to the quality of the light, coloured or colourless, diffused through plain or coloured glass. _Stained Glass Windows._ 1. From the bad effect of the mutual proximity of white and stained glass (365), it results that where one is employed in a church the other must be excluded, at least from the nave, choir, in a word, from all that the spectator can embrace at one point of view. The colourless glass in some of the chapels of the aisles is of no consequence in the general effect. 2. If pictures be near stained windows, they must be flat, or present subjects as simple as possible, since their effects are entirely sacrificed to those of the stained glass (438). 3. We can place pictures in a large church where the light is transmitted through coloured glass; but, for the view to be satisfactory, they must necessarily encounter such a union of conditions, that they will almost always be found out of place. In fact, if the pictures are not at a certain distance from the glass,—if the coloured lights which emanate from them are not, by their mutual admixture, in the requisite proportions for producing white light, or, at least, a very faintly coloured light—or if this feebly coloured light is insufficient to lighten the interior of the church properly, the pictures will lose their colour, unless they have been executed with reference to the nature of the light from the stained windows; but this is not, to my knowledge, ever realized. _White Glass Windows._ 454. Churches with white glass windows harmonize with every ornament we can imagine in the employment of wood, marbles, porphyry, granite, and the metals. Mosaics may ornament the floors and adorn the walls with true pictures, as we see in St. Peter’s at Rome. Painting in fresco, in oil, plain and coloured sculptures, also combine to ornament the interior. 1. In churches of this class, the profusion of riches at the disposal of the decorator, far from being always of advantage to him, may be the cause of difficulties; for, the more varied the objects he has to arrange, the greater the difficulty of presenting only such objects as are in keeping with the place he has to embellish. It is not enough to have precious woods, marbles, metals, pictures; he must also make these objects harmonize. Thus, he must avoid putting coloured marbles contiguous to the white stone of which the walls are constructed; he must also proscribe surrounding bas-reliefs in white stone with slabs or borders of red or green marble.