white rays exert an intermediate action. A long series of experiments on birds satisfied Béclard that the quantity of carbonic acid thrown out in breathing, during a given time, is not sensibly modified by the different colors of the glasses the animals are placed under. It is the same with small mammifers, such as mice; but it is to be observed in this case that the skin is covered either with hair or feathers, and the light does not strike the surface. The same physiologist examined also the influence of the different-colored rays of the spectrum on frogs. Under the green ray, the same weight of frogs produces in the same period of time a greater quantity of carbonic acid than under the red ray. The difference may be a half greater; it is usually a third or a fourth greater; but if the skin is afterward taken off the frogs, and they are replaced under the same conditions, the result alters. The amount of carbonic acid thrown out by the flayed frogs is greater in red than in green light. A few experiments tried by Béclard on the exhalation of the vapor of water by the skin show that in the dark, temperature and weight being alike, frogs lose by evaporation a half or a third less moisture than under white light. In the violet ray the quantity of moisture lost by the animal is perceptibly the same as in white light.
Light acts directly on the iris of almost all animals, and thus produces contraction of the pupil, while heat produces the reverse phenomena. This stimulus is observed in eyes that have been separated for some time from the body, as Brown-Séquard has shown.
Bert lately took up some very curious experiments on the preference of animals for differently-colored rays. He took some of those almost microscopic Crustacea, common enough in our fresh waters, the daphne-fleas, remarkable for their eager way of hurrying toward light. A number of these insects were put into a glass vessel, well darkened, and a spectrum of the ray then thrown into it. The daphnes were dispersed about the dark vessel. As soon as the spectrum-colors appeared, they began to move, and gathered in the course of the luminous track, but, when a screen was interposed, they scattered again. At first all the colors of the spectrum attracted them, but it was soon noticed that they hurried much more toward the yellow and green and even moved away a little if these rays were quickly replaced by the violet. In the yellow, green, and orange parts of the spectrum there was a thronging and remarkable attraction. A pretty large number of these little beings were remarked in the red, too, a certain number in the blue, and some, fewer in proportion to the distance, in the most refrangible portions of the violet and ultraviolet. For these insects, as for ourselves, the most luminous part of the spectrum was also the most agreeable. They behaved in it as a man would do who, if he wished to read in a spectrum thrown about him, would approach the yellow and avoid the violet. This proves, in the first place, that these insects see all the luminous rays that we see ourselves. Do they