570 Wisconsin Academy of Sciences, Arts, and Letters. 
and a distortion of the image with change of color resulted. 
By putting set-screws on the tube, the plates were made ad¬ 
justable and set accurately parallel by an optical method. 
4. Keadings on Distilled Water 
The readings first taken were upon distilled water so that 
a comparison could be made with the results of previous in¬ 
vestigators. Distilled water was used throughout the prelim¬ 
inary work of refining the apparatus and the coefficients given 
in Table 1 indicate the final results obtained. The results 
of Hiifner ai?d Albrecht,^ Ewan,^ Aschkinass,^ and von Auf- 
sess^ are also recorded. The values are given for the absorption- 
coefficient, A, as defined by the equation 
J' = Je-A<i 
where d = 1 m. 
In all of the tables and diagrams the absorption of light is 
stated in terms of the coefficient of absorption per meter. 
This is the reciprocal value of the thickness of the stratum 
of water which would reduce the intensity of the transmitted 
light to the fraction cT^v^of its value at entrance. The 
Z. < loo 
number 2.7183 is the basis of the natural system of logarithms 
and the value of the fraction is approximately 0.37. This co¬ 
efficient may be stated in terms of any desired unit of measure 
for the thickness of the stratum. In this case the meter is 
employed. An absorption coefficient of unity, therefore 
means that a stratum of water whose thickness is unity (1.000 
meter) absorbs so much light that the intensity of the trans¬ 
mitted light is equal to 0.37 of its original value. An absorp¬ 
tion coefficient of .200 means that if the ray of light were passed 
through a layer of water 5 meters thick ( ) its intensity 
would be reduced to 0.37 of its original value. Similarly a 
coefficient of 1.250 would mean that 0.80 meters would reduce 
the intensity of transmitted light to 0.37 of its original value. 
1 Hiifner and Albrecht, Wied. Ann. d. Phys. u. Chemie, VoL 42, pp. 1-17, 
1891. 
2 Ewan, Proc. R. Soc. 57, p. 117-161, 1894. 
^Aschkinass, Wied. Ann. d. Physik. und Chemie, Vol. 55, p. 401, 1895, 
^Aufsess. Die Farbe der Seen, Inaug-Diss. Miinchen, 1903. 
