Pietenpol—Visible Spectrum of Wisconsin Lake Waters 593 
near Trout lake; they are partly from lakes, Turtle lake in 
the northern part of the state, and North lake (1914), a mem¬ 
ber of the Oconomowoc-Waukesha group. In these six waters 
the more deeply stained show a steady decline in the co¬ 
efficient of absorption from 5750A on and consequently an ap¬ 
proach to the absorption of pure water. The less deeply 
stained show an absorption which is practically stationary 
through this part of the spectrum. The ‘‘swamp” water 
shows a phenomenon quite parallel to that of Waubesa; from 
6180 a on the absorption declines and at 6630 a is practically 
identical with that of Marl lake—one of the clearest waters— 
and is but little above that of pure water. 
The case of Eound lake deserves a word to itself (see fig. 13). 
This is one of the Waupaca lakes and is of the marly type. Its 
water is of the same type as those of Green, Marl, etc., but with 
a far higher amount of absorption. This is true for both 
years in which it was observed. It constitutes a unique and, 
so far as present data go, an inexplicable ease among the lakes 
studied. 
These notes show part of the limnological conclusions which 
may be drawn from Dr. Pietenpol’s data. It is plain that the 
observations on which they are based constitute the beginning 
of a large investigation rather than its completion. They 
show that the study of lake waters by the spectrophotometer 
may enable us not only to group them into large classes, as 
von Aufsess did, into “blue lakes,” “green lakes,” etc., but 
also to classify them more accurately according to the various 
kinds and mixtures of stains which color the water and which 
give rise to different forms of absorption curves. The 
observations here recorded indicate at least one well marked 
variety of the yellow-brown waters—that which I have called 
the marsh-stained—and also that this type of curve differs 
widely from that offered by other types of lakes with colored 
waters. 
These notes, however, indicate rather the possibilities of 
such a study than any final or complete results. This Sur¬ 
vey is not likely to carry the investigation further at present, 
since it is a slow and laborious one and the data already ob¬ 
tained are sufficient for the primary purpose with which it was 
undertaken—that of ascertaining the general facts regarding 
the absorption of the sun's energy by the surface strata of 
lakes. 
38~s. A. L. 
