Pietenpol—Visible Spectrum of Wisconsin Lake Waters 581 
water. This conclusion was based at first on the experience of 
von Aufsess (’03, p. 30) who found that suspended matters did 
not alter the form of the absorption curve of the lake. It was 
confirmed by Pietenpol’s comparisons of filtered and unfiltered 
water, some of which are recorded on p. 574. • 
There is, therefore, apparently a fundamental difference be¬ 
tween the observations of von Aufsess and Pietenpol, one set 
being made on the lake itself and the other on filtered lake 
water. But in fact the results of the two methods are com¬ 
parable, except in one particular. The particles in suspension 
decrease the amount of light transmitted though they do not, 
in general, alter its color. The position of the curve of absorp¬ 
tion is changed in the two methods, but not its form. This 
must be kept in mind in comparing von Aufsess’ curves with 
those of this paper. Since the amount of suspended matter 
is a very variable one, the degree of weakening of the trans¬ 
mitted light due to it, is correspondingly variable; and since 
the investigation concerns primarily the color rather than the 
total quantity of transmitted light, it is an advantage to elimin¬ 
ate the suspended particles. It is certain, for instance, that in 
von Aufsess’ observations every coefficient of absorption has 
been determined by two kinds of factors, first the selective ab¬ 
sorption of water and stain, and second, the non-selective action 
of suspended particles. It is impossible to tell how much is 
attributable to each of the factors. In Pietenpol’s method, the 
selective absorption alone is considered, and the non-selective 
absorption can readily be determined by other methods if it is 
desired. In any case, the spectrophotometer is not a good in¬ 
strument for the latter purpose. 
Suspended matters may greatly modify the color of lakes as 
seen from the shore. The apparent color of the water is af¬ 
fected by the light reflected from particles in suspension. 
Marly lakes have a peculiar bluish-green tinge, due in part to 
light reflected from the white particles of marl. Lakes whose 
plankton is abundant acquire a brownish shade when a crop of 
diatoms comes up, and a different color when filled with 
Ceratium. These influences are eliminated by the process of 
filtration. But they are also eliminated by any scheme which 
determines the' color of a lake by means of light transmitted 
through the water, either directly, or (as in von Aufsess’ ease) 
reflected from a white disc. However this process may be 
