Pietenpol—Visible Spectrum of Wisconsin Lake Waters 587 
high absorption in the blue end of the spectrum, for which it 
was devised. 
The color of the water is due in all cases to extractive matters 
and these are numerous, complex, and vary not only in amount, 
but also in kind and number from lake to lake, and in the same 
lake from time to time. No one scale or standard can, there¬ 
fore, represent them with accuracy—as von Aufsess showed 
for the methyl blue and potassium chromate scales (’03, p. 
36-38.) 
Fig". 11. Absorption curves of lakes rated 18-22 on the color scale: 
1. Lac la Belle; 2. Oconomowoc lake; 3. Okauchee lake; 4. Lake 
Waubesa. 
In general, therefore, the water of the following lakes seems 
to have a kind of color which the spectroscope shows to depend 
on a mixture of similar stains, and the difference in intensity 
seems to depend essentially on the quantity of absorption due 
to these stains in the region toward the blue end of the spectrum 
