Pietenpol—Visible Spectrum of Wisconsin Lake Waters 567 
and since the intensity is proportional to the square of the 
amplitude, 
~ z=: tan^ 6 . 
Jb 
The angle 6 is obtained from the reading of the circular scale 
of the analyser, when the reference point Jb “ 0 has previously 
been determined. 
Fig. 2. See explanation in text. 
If the intensity of a comparison ray is measured against 
that of a ray which passes through an absorbing medium the 
following relation exists, 
= tan^ 6^1 
In comparison now with the ray when it has passed through a 
different thickness of the absorbing material. 
^ = tan^ $2 
and. 
J 2 _tan^ 
J1 tan^ $2 
The coefficient of absorption. A, is defined by the equation. 
J' = J e-Ad 
where J is the intensity of the oncoming light, J' is the result¬ 
ing intensity, and d is the thickness of the absorbing medium. 
As a result there exists the relation. 
e-Ad 
tan^ (9i 
tan^ $2 
and. 
A = 
tan^ $2 
tan^ 
