230. There are two modes in which any mental or material effect may be increased--by contrast, or by assimilation. Supposing that we have a certain number of features or existences under a given influence; then, by subjecting another feature to the same influence, we increase the universality, and therefore the effect, of that influence; but by introducing another feature, _not_ under the same influence, we render the subjection of the other features more palpable, and therefore more effective. For example, let the influence be one of shade, to which a certain number of objects are subjected. We add another feature, subjected to the same influence, and we increase the _general impression_ of shade; we add the same feature, _not_ subjected to this influence, and we have deepened the _effect_ of shade. Now, the principles by which we are to be guided in the selection of one or other of these means are of great importance, and must be developed before we can conclude the investigation of villa architecture. 231. The impression produced by a given effect or influence depends upon its degree and its duration. Degree always means the proportionate energy exerted. Duration is either into time, or into space, or into both. The duration of color is in space alone, forming what is commonly called extent. The duration of sound is in space and time; the space being in the size of the waves of air, which give depth to the tone. The duration of mental emotion is in time alone. Now in all influences, as is the degree, so is the impression; as is the duration, so is the effect of the impression; that is, its permanent operation upon the feelings, or the violence with which it takes possession of our own faculties and senses, as opposed to the abstract impression of its existence, without such operation on our own essence. For example, the natural tendency of darkness or shade is to induce fear or melancholy. Now, as the degree of the shade, so is the abstract impression of the existence of shade; but as the duration of shade, so is the fear or melancholy excited by it. Consequently, when we wish to increase the abstract impression of the power of any influence over objects with which we have no connection, we must increase degree; but, when we wish the impression to produce a permanent effect upon ourselves, we must increase duration. Now, degree is always increased by contrast, and duration by assimilation. A few instances of this will be sufficient. 232. Blue is called a cold color, because it induces a feeling of coolness to the eye, and is much used by nature in her cold effects. Supposing that we have painted a storm scene, in desolate country, with a single miserable cottage somewhere in front; that we have made the atmosphere and the distance cold and blue, and wish to heighten the comfortless impression.