A common division of the spectrum is into warm and cold colors. The warm colors are red, yellow, orange, and yellow-green; the cold colors are violet, blue, green, and blue-green. This is not an arbitrary division, but answers to a fact of experience which passes from our physical to our moral impressions, and may cause in us feelings of comfort or uneasiness, joy, sadness, or moral depression. Some persons are influenced by the gray-colored sky, others are gay when the day is bright. It is a current expression that the color of the southern landscape is warm. Goethe said that blue caused him to feel cold. The terms warm and cold are technical expressions in the arts. A color tone is cooled by putting blue in it, and warmed by adding red or yellow. "This practice is not arbitrary," says M. F. Bracquemond in his book on Design and Color; "it copies the colored aspects which natural light imposes on all imitation that seeks to realize the colored and factitious light of painting. To reach this, art observes the order according to which the natural lights distribute their various colored elements, and classes luminous aspects--a process which it has always observed--into the two categories of warm and cold. Hence, so far as examples come to us, this contrast is easy to verify; at the Louvre, for example, in works from Pompeii, and in those of all the masters." Preyer relies upon this division of colors into warm and cold for a comparison of chromatic sensations with thermic, and for supposing that the color sense is developed from the sense of temperature. Chromatic sensitiveness to this author is only a special case of thermic sensitiveness limited to the retina. Darwin's ideas were evidently the same; the whole human body was a sort of retina capable of improvement; we may, it is true, suppose with Lord Kelvin that "there is absolute continuity between the perception of heat by the retina of the eye and its perception by means of the tissues and nerves." A very elementary experiment will easily enable us to recognize these different qualities of colors. Set a lighted candle on a table near a window; there are then two sources of light--the daylight, blue and cold, and the light of the candle, orange-red and warm. Cast a shadow on the white paper by holding a pencil straight up. The shadows cast by the candle will be blue to a degree that no one can mistake it, a greenish blue. Placing the pencil between the window and the candle and looking at the shadows, we have, first, the blue shadow of the candle, and then the shadow projected by the cold daylight. The color of the last, though perhaps less evident than the other, is an orange-yellow, of rich, warm tone.