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Durand, Loyal, Jr. (ed.) / Transactions of the Wisconsin Academy of Sciences, Arts and Letters
volume XXXI (1938)

James, Harry Raymond; Birge, Edward A.
A laboratory study of the absorption of light by lake waters,   pp. [1]-154 PDF (46.6 MB)

Page 23

 James & Birge—Lake Waters and Light 23 
If the two-meter tube has been used, the calculation of per cent absorption
per meter is conveniently done by first calculating the absorption coefficient
(A) of the equation I = L e~ where (Jo) is the energy transmitted through
the comparison cell, (I) is the energy transmitted through the water tube,
(x) is the length of the tube in meters, and (e) is the base of the Napierian
system of logarithms. From the above relation 
~ (loglo—logl) 
and since the galvanometer deflections (d0) and (d) are tested to be proportional
to the energies (Ia) and (I), the deflections 
may be substituted directly in place of the energies and the value of (A)
is given by 
 A = (logdo—logd) 
Method of Presenting Results 
 The result of observation at any designated wave-length is stated in terms
of the percentage of radiation which is absorbed by one meter of the lake
water which is under examination. The percentages thus obtained are used
as ordinates, with wavelengths as abscissas, to plot percentile absorption
curves, from which come the numerous diagrams of Part I. These are intended
to present to the eye the effect of these waters on the spectrum. 
 The lake waters have yielded some 3650 such percentages, which are recorded
in Tables X, XI, XII, pp. 100-109; these tables contain the data on which
the report is based. 
 The curves of the diagrams necessarily show both the percentile absorption
of radiation by a meter of water, and its complement, the percentile transmission.
Absorption is emphasized in the make-up of the diagrams, since such results
have commonly been expressed in terms of coefficients of extinction. This
Survey has found the use of percentages more convenient than that of coefficients.
 In computing the ordinates for the various types of curves discussed in
Chapter IV, recourse is had to percentile transmission; but it has rot been
necessary to alter the character of the diagrams. Fig 58, p. 146, offers
a diagram as a specimen of per- 

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