From Hensleigh Wedgwood [1867–72]1
I think that a brown study is merely another version of the same metaphor which speaks of a sombre countenance, ein düsteres gesicht2 Heiter and düster express chearful and moody as being open to light and external influences or closed and shut up within itself.3
Expression: derivation of the term "brown study".
Please cite as
Darwin Correspondence Project, “Letter no. 7042,” accessed on 27 April 2017, http://www.darwinproject.ac.uk/DCP-LETT-7042
Also published in The Correspondence of Charles Darwin, vol. 20 (Expression supplement)