LIGHT AND LIFE. 119 



duces in the same period of time a greater quantity of car- 

 bonic 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 Be"clard 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 pro- 

 duces the reverse phenomena. This stimulus is observed 

 in eyes that have been separated for some time from the 

 body, as Brown-Se"quard 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 gath- 

 ered 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 



