Darwin was quite experienced with the microscope. In this experiment we see another aspect of the “experimental Darwin”. Here Darwin is examining the effect of ammonia on plants (this is part of his research in insectivorous plants)
When I saw you I had tried only putting a minute drop on the leaves on growing plants and observing whether or not they contracted as if over a fly.f3 From many reasons I inferred that the leaves absorbed some nitrogenous element, probably some form of Ammonia; so I thought I would try under the microscope the effect of C. of Ammonia; and several other salts and substances, and it is truly wonderful how quickly a minute dose acts and produces marvellous changes in the absorbing glands and in the adjoining cells. I have tried plain water over and over again with no effect. I have over and over again kept a leaf for 1, 2, 3, 4, and 5 hours in water with no effect, and then put these same leaves in a few measured drops of very weak solutions of C. of Ammonia (all made, and re-made by myself) and the same peculiar effects were produced in one hour or 11⁄2 hour as is produced instantaneously by a stronger solution. Generally I have scrutinized every gland and and hair on leaf before experimenting; but it occurred to me that I might in some way affect the leaf; though this is almost impossible, as I scrutinized with equal care those that I put into distilled water (the same water being used for dissolving the C. of Ammonia). I then cut off 4 leaves (not touching them with fingers) and put in plain water and 4 other leaves into the weak solution and after leaving them for 11⁄2 hours I examined every hair on all 8 leaves; no changes on the 4 in water every gland and hair affected in those of Ammonia. [bold mine - afarensis]
I had measured the quantity of weak solution and I counted the glands which had absorbed the ammonia and were plainly affected; the result convinced me that each gland could not have absorbed more than 1/64,000 or 1⁄65000 of a grain.— I have tried numbers of other experiments all pointing to the same result. Some experiments lead me to believe that very sensitive leaves are acted on by much smaller doses.
Basically, Darwin is setting up controls for the possibility that he somehow contaminated the results.
Darwin Correspondence Project Database. http://www.darwinproject.ac.uk/entry-2973/ (letter no. 2973; accessed 15 January 2010)