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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
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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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