LIGHT AND LIFE. 119 
duces in the same pericd 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 ora 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 pro- 
duces 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 thrownintoit. 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 
