580 Wisconsin Academy of Sciences, Arts, and Letters. 
Coefficient of Absorption in Various Lake Waters 
Notes on the Foregoing Paper 
E. A. Birge 
In the foregoing report, Dr. Pietenpol has stated his results 
rather in the terms of a physical experiment than in those of 
limnology. I, therefore, add a few notes bearing on the lim¬ 
nological results obtained from them. 
The investigation was carried out by Dr. Pietenpol at my 
request. Its results serve two ends; first, they show the ab¬ 
sorption of light in Wisconsin lakes by colored matters dissolved 
in the water, and, second, they offer one part of the data by 
which the absorption of solar energy in the surface strata of 
lakes may be distributed, approximately, among its three chief 
factors—the selective absorption of water itself, selective ab¬ 
sorption due to color of water, and absorption due to susp\'nded 
particles. The latter purpose was the more important one, 
but is not considered in this paper. 
The first, and thus far, I believe, the only student to apply 
spectrophotometric methods to lakes, was von Aufsess.^. By 
means of a skillful adaptation of the instrument to use in a 
boat and with great labor, he studied the water of ten Bavarian 
lakes in situ. He determined their absorption curves and he 
classified the lakes, as a result of his observations, into four 
types; 1, blue; 2, green; 3, yellow-green; 4, yellow or brown. 
He also reached conclusions regarding the nature of the color 
of pure water and other matters, which do not concern us here. 
Observation with the spectrophotometer on the lake itself 
demands so many simultaneous favorable conditions that good 
opportunities are rare. In the case of the Walchensee, von 
Aufsess had to wait four weeks for his observations. This dif¬ 
ficulty is very likely one reason why no one has followed him 
along so promising a line of study. 
It was plain that methods dependent on such exceptional 
weather conditions were not at all adapted to reach the ends 
sought by this survey. It was determined therefore to work in 
the laboratory, not on the lake; and also to make use of filtered 
^ von Aufsess, O. Die Farbe der Seen. Inaugural Dissertation, Munich. 
1903. 
