The maximum depth of the limit of visibility as found by Prof. Forel, in
Lake Geneva, was 56 feet. He showed, moreover, that this limit is much
greater in. winter than in summer, as explained in part by the greater
absence of suspended matter and in part by the fact that increase of
temperature increases the absorbing power of water for light. The
maximum depth of visibility in the Atlantic Ocean, as found by Count
de Pourtales, was 162 feet, and Prof. Le Conte states his belief that
winter observations in Lake Tahoe would place the limit at even a
greater depth than this. The author gives a detailed and interesting
discussion in regard to the blue color of lake waters, reviewing in full
the results of previous writers on the subject, and concludes that while
pure water unquestionably absorbs a larger part of the red end of the
spectrum, and hence appears blue by transmitted light, the color seen by
diffuse reflection is mainly due to the selective reflection from the
fine particles suspended in it.
The last subject discussed by the author is that of the rhythmical
variations of level, or "seiches," of deep lakes; he applies the usual
formula to Lake Tahoe, and calculates from it the length of a complete
longitudinal and of a transverse "seiche;" these are found to be
eighteen or nineteen minutes in the first case and thirteen minutes in
the second.
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