576 Wisconsin Academy of Sciences^ Arts, and Letters, 
determined. - The results are given in Table 3. The curves 
in Fig. 6 show graphically the relation between the color ab¬ 
sorption after different periods of time have elapsed. In the 
curve taken Dec. 4, 1915 a drop in the yellow region of the 
spectrum indicates the slight loss of color of a dark brown 
water with time. 
TABLE 3 
EFFECT OF TIME ON COEFFICIENT OF ABSORPTION 
Turtle Lake Water, Samples Taken Sept. 3, 1915 
Wave length 
Sept. 17, ’15 
Oet. 18, ’15 
Dee. 4, ’15 
5220... 
1.900 
1.770 
1.644 
5390...... 
1.580 
1.420 
1.272 
5580.... 
1.300 
1.210 
.995 
5790. 
1.070 
1.043 
.865 
5895. 
1.015 
.990 
.836 
6005. 
.958 
.94r7 
.832 
6180. 
.925 
.890 
.804 
6360. 
.879 
.827 
.765 
6480... 
.859 
.806 
.7a3 
6630... 
.828 
.777 
.730 
7. Effect of Depth on Color 
Observations to determine the effect of depth upon the color 
of water were also made. It was noticed, in the case of lake 
Mendota, that water taken from a considerable depth some¬ 
times appeared to be slightly more highly colored than the 
surface water. On July 15, 1915 a determination of the selec¬ 
tive absorption of Mendota water taken from the surface and 
from a depth of twenty meters was made. The results for this 
ihstance as given in Table 4 show that the absorption by the 
surface water was greater than that by the deep water through¬ 
out the yellow portion of the spectrum, while in the red the 
reverse relaxation existed. It is possible that greater differences 
might have been found had water been taken later in the 
season. In the data which are recorded the wave-lengths are 
given in Angstrom units. 
