562 Wisconsin Academy of Sciences, Arts, and Letters. 
SELECTIVE ABSORPTION IN THE VISIBLE SPECTRUM 
OE WISCONSIN LAKE WATERS 
W. B. PlETENPOL 
Notes from the Laboratory of the Wisconsin Geological and 
Natural History Survey. XII. 
1. Introduction 
When light is transmitted through a material medium some 
of the radiant energy is absorbed, that is, transformed into 
another form of energy, as heat, chemical energy, or some other 
frequency of radiation. All bodies, even the most transparent 
for visible light as water, glass, quartz, etc., possess this property 
of absorption. A body is colorless by transmission when it 
transmits the same percentage of radiation for every wave¬ 
length or frequency. On the other hand a body may show 
selective absorption and the color of the body is determined 
by the wave-length of that radiation of which the greatest per¬ 
centage is transmitted. A determination of the selective ab¬ 
sorption in the visible spectrum of distilled water and the 
waters of some Wisconsin lakes, giving the relation between 
the decrease of intensity and depth of penetration as a func¬ 
tion of wave-length, was the object of this investigation. 
In connection with the work on limnology, carried on by Dr. 
E. A. Birge and Mr. C. Juday of the Wisconsin Geological 
Survey, it was of interest to ascertain the rate of absorption 
of light energy by lake waters. This has a direct influence 
upon the temperature of the water and also upon the chemical 
action which takes place beneath the surface of the lake. 
Recent experiments with color filters have shown that the 
physico-chemical changes in plant life which result from light 
