Pietenpol—Visible Spectrum of Wisconsin Lake Waters 583 
Marl lake furnishes the most transparent water, which is much 
like that of Walehensee. At' the other end of the scale, Turtle 
lake is much more highly colored than Arbersee, the most 
deeply tinged of those examined by von Aufsess. For these 
curves see table 5 and fig. 7, and compare them with von Aufsess' 
tables and plates. 
Illustrations of curves from Wisconsin lakes are given in fig. 
7 in which are platted typical curves from the series of lakes, 
ranging from the clearest water to the most highly colored. 
No lake in the series shows the smallest absorption at the blue 
end of the curve. None of them, therefore, can come into von 
Aufsess' class of ‘^blue lakes''^ (’05. p. 89) although Marl lake 
comes very close to it. In most cases of the clear waters, how¬ 
ever, there is a minimum of absorption about 5400-5600 A, in 
the green, and the water would therefore agree with von 
Aufsess’ second type—the ''green lakes.” In by far the great 
majority of the Wisconsin lakes, the water is decidedly stained 
by dissolved coloring matters, and there is a strong increase of 
absorption toward the blue end of the spectrum. The water 
has, therefore, a color varying from yellowish green to brown 
according to the depth of the stain. 
In the most highly colored waters, like that of Turtle lake, 
there is no minimum of absorption in the yellow or green, or at 
most only a hint of such a minimum. The blue is wholly 
absorbed and the coefficient of absorption falls off rapidly 
through the green to about 5800 A, and much more slowly at 
greater wave lengths. 
It is, therefore, obvious that all the Wisconsin waters are 
more or less stained with extractive matters. An attempt was, 
therefore, made to correlate the selective absorption with the 
color as ascertained by the use of the United States platinum- 
cobalt scale^—a scale devised especially for use on such waters.^ 
The water of 21 lakes was examined in 1915 with results as 
shown in the following table: 
1 von Aufsess, O. Die physikalischen Eigenschaften der Seen. Braun¬ 
schweig, 1905. 
^ Leighton, M. O., Field Assay of "Water. "U. S. G. S. Water-supply 
Bull., Series L, No. 11. Washington, 1905. p. 42. 
