Pietenpol—Visible Spectrum of Wisconsin Lake- Waters 563 
action^ depend up the quality as well as the intensity of the 
light incident upon the percipient region. Electrons affected 
differently by the action of light of one wave-length and an¬ 
other are the cause of the chemical actions from which results 
the building up of complex organic molecules. The chlorophyl 
of the plant has been found to be more sensitive to the blue^ 
or light of short wave-length than to that of long wave-length. 
So the absorption spectra of the different lake waters should 
have a direct bearing upon the nature of the planktonic life 
found in these lakes. With such an incentive work was be¬ 
gun in the Physical Laboratory of the University of Wisconsin 
during the summer of 1914 and continued during the two years 
following. The results of this investigation we hope may be 
of value to the limnologist. 
2. General Statement 
Since 1882^ the selective absorption theory has been quite 
generally recognized as explaining the color of water. In op¬ 
position to this, the theory of selective reflection had attributed 
the color to light reflected from extremely small particles held 
in suspension. The effect of such small particles in scattering 
incident radiation has been fully discussed by Lord Kayleigh, 
who based upon this his mathematical theory of the blue color 
of the sky. When the particles of matter are large compared 
with molecular dimensions the luminosity is reduced fairly 
> equally throughout the range of the visible spectrum, and there 
would be no great change in the relative intensities of red and 
blue. When, however, the particles are of molecular dimen¬ 
sions the scattering in the extreme violet is sixteen times as 
great as in the extreme visible red. An approximation of 
Eayleigh's formula shows that the intensity of the scattered 
light varies inversely as the fourth power of the wave-length. 
The general theory of radiation scattered by a spherical ob¬ 
stacle has led to an extensive study of colors exhibited by col¬ 
loidal solutions, metal glasses, and metallic films. Mie^ in his 
electromagnetic consideration of scattered radiation shows 
curves for absorption in colloidal gold solutions and it may be 
of interest to note that for particles of size IGO/x/x the curve 
shows a resemblance to the absorption curve of distilled water. 
^ Mie. Aim. der Phys. Vol, 25, p. 377, 1908. 
