Pietenpol—Visible Spectrum of Wisconsin Lake Waters 569 
meter. The tube was paraffin lined and had a length of 320 cm. 
and a diameter of 5 cm. Emerging from the tube containing the 
absorbing solution, the ray was brought to a fine focus upon the 
ground glass front plate, Gr, of the spectrophotometer by the 
combination of achromatic lenses Lg. This image was used 
as the effective slit-width and sent into the slit of the photo¬ 
meter, Sj. The comparison ray reflected from the plate glass 
P passed successively through the total reflecting prisms, 
and Rg. It was then brought to a focus in about 11 cm. by 
the combination of achromatic lenses, L 3 . By means of the 
total reflecting prism R 3 it was sent into the slit, S 2 , with the 
same convergence as the direct ray which enters S^. 
Fig. 3. Arrangement of apparatus for observations on water. 
For the determination of the absorption coefficient two runs 
are necessary, one when the comparison ray is set against the 
ray "which passes through the tube, T, of very short length 
and the other when the tube is of full length. By this means the 
difference between the reflection of glass-water and glass-air sur¬ 
faces, which would exist if readings were taken with the tube 
empty and then filled with water, are eliminated. A difficulty 
which was encountered in first testing out the apparatus was the 
inability to make a satisfactory check for the coefficient of ab 
sorption of water when different lengths ol tubing were used. 
With a wide beam of parallel light almost equal to the inner 
diameter of the tube, focussed upon the slit of the spectrophoto¬ 
meter there was apparently a drop in the violet portion of 
the absorption curve with increased length of water column. 
This seemed satisfactorily explained by the non-achromatism^ 
of the lenses first used in addition to the effect the column of 
water might have, since the violet rays are more refrangible 
than the red. It may possibly explain the low values foimd 
by von Aufsess (’03, p. 26. See also table 1 of this paper.) in 
the short wave-length region of the spectrum. 
When the glass end plates of the tube were not accurately 
parallel, the column of water acted as a prism of small angle 
