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Darwin Correspondence Project

From F. C. Donders   12 July 1874

Marienbad (Bohemia)—

12 July 1874.

My dear Mister Darwin.

As your son George, whom I was very happy to see, told me of your experiments on Drosera, I, in fact, mentioned the great sensibility of the muscular fibres of the iris for atropine.1

It is 25 years ago that I made experiments on this point, with one of my pupils (Dr. de Ruyter)2 and I repeat a part of them every year on my lectures.

The question was, whether the atropine (sulphate of atropine has been used)3 acts directly or indirectly on the iris. We found, that it is a direct action. Small doses act only by application on the cornea, and the atropine is transported into the eye by imbibition. We applied repeatedly on the cornea of a rabbit a solution of 1 to 100 water, washed thoroughly out the conjunctival sac with a stream of water, evacuated by a punction4 the aqueous humour and put it in the eye (on the cornea) of a dog, whose iris is very sensible to atropine: it really produced a notable dilatation of the pupil for some hours.

The question now was this, how much atropine the aqueous humour of the rabbit contained. By comparing with a series of dilutions of our normal solution, we found, that a solution of 1 to 100000 acts as much on the dog’s eye as the aqueous humour of the rabbit’s eye, after the application of 1 to 100.— We conclude, that after an application of 1 to 100, the real acting solution (the aqueous humour) is at 1 to 100,000. Now, after an instillation of 1 to 100000, the acting solution may be of 1 to 100,000000. Of course, the action continues for a certain time, and whilst some particles are absorbed by the internal vessels, other arrive from the cornea into the aqueous. But certainly, we go not too far, if we admit, that alltogether not 1:1.000000 of a grain enters and acts on the iris, and that the acting solution contains, ad maximum,5 less than 1 to 10.000000

And still, we know, that the dog’s iris is a rather large organ.— Perhaps I can send you a copy of de Ruyter’s dissertation, written in Latin, as soon as I am back in Utrecht.6

You are aware that the action of atropine is chiefly paralysing (perhaps irritant on the radiary fibres). Irritant, in the sense of phosphate of Ammonia, is the action of Physostigmine (from the Calabar bean, Physostigma venenosum),7 but it seems to me, that it is not quite so active in minimal doses as the atropine: I did not determine it in numbers.

Perhaps the sensibility of the anterior grey substance of the medulla spinalis8 of the frog for the direct application of strychnine9 (producing tetanus in the corresponding muscles) is as great as that of atropine on the Iris

Professor Engelmann, my son in law, published a book on the influence of several substances, especially of vapours and gazes, on vibratile motion, on the motions of spermatozoids of protoplasma etc. and I remember that he found a wonderfull sensibility for some of them. He will be very happy to be allowed to send you a copy (in German), after having had the opportunity to signalise in the copy the facts, which may be of some importance to you.10

I was very happy to hear from your son, that your health is tolerably good, and it was quite an honour and a pleasure to me to receive your letter. Here I had nothing better to do than to answer immediately. Most willingly I will supply to you afterwards a more complete account of the facts, if you want it.

Pray, believe me with the highest esteem | Yours very sincerely | Donders.

addresse: | Prof. D., zum | Telegrafen, | Marienbad.

CD annotations

3.2 directly … on the iris.] underl red crayon
3.2 direct action] underl red crayon
4.3 1 to 100000] underl red crayon
4.3 100000] altered red crayon to ‘100,000’
4.9 But certainly … 10.000000 4.10] ‘Estimation’ added red crayon
4.10 and that … 10.000000 4.11] scored red crayon
4.11 10.000000] altered red crayon to ‘10.000,000’
9.1 I was … want it. 9.4] crossed pencil


See letter to F. C. Donders, 7 July 1874. George Howard Darwin had visited Donders in Utrecht (letter from Emma Darwin to Leonard Darwin, 7 July 1874 (DAR 239.23: 1.19)). Drosera is the sundew.
Gerardus Cornelius Petrus de Ruiter.
Atropine is an alkaloid substance derived from some plants of the family Solanaceae. Atropine sulphate is a common atropine compound used in medicine.
Punction: a prick; a puncture (OED).
Ad maximum: at most (Latin).
Donders refers to Ruiter’s Dissertatio physiologico-medica de actione atropae belladonnae in iridem (Physiological-medical dissertation on the action of Atropa belladonna on the iris; Ruiter 1853). Atropa belladonna (deadly nightshade) is a plant from which atropine is derived. Ruiter also published an Dutch version of his dissertation in the Nederlandsch Lancet (Ruiter 1853–4).
Physostigmine, an alkaloid substance derived from the seeds of Physostigma venenosum (the calabar bean), inhibits the transfer of signals from nerve cells to muscles.
Medulla spinalis: spinal cord (Latin).
Strychnine is an alkaloid substance commonly derived from the seeds of Strychnos nux-vomica (the strychnine tree); it stimulates the motor ganglia of the spinal cord.
Theodor Wilhelm Engelmann’s Über die Flimmerbewegung (On ciliary movement; T. W. Engelmann 1868) focussed on external chemical, electrical, and physical causes of change in vibratile motion.


Engelmann, Theodor Wilhelm. 1868. Über die Flimmerbewegung. Leipzig: Wilhelm Engelmann.

OED: The Oxford English dictionary. Being a corrected re-issue with an introduction, supplement and bibliography of a new English dictionary. Edited by James A. H. Murray, et al. 12 vols. and supplement. Oxford: Clarendon Press. 1970. A supplement to the Oxford English dictionary. 4 vols. Edited by R. W. Burchfield. Oxford: Clarendon Press. 1972–86. The Oxford English dictionary. 2d edition. 20 vols. Prepared by J. A. Simpson and E. S. C. Weiner. Oxford: Clarendon Press. 1989. Oxford English dictionary additional series. 3 vols. Edited by John Simpson et al. Oxford: Clarendon Press. 1993–7.

Ruiter, Gerardus Cornelius Petrus de. 1853–4. Onderzoekingen over de werking van Atropa Belladonna op de iris. Nederlandsch Lancet (1853–4): 434–72.

Ruiter, Gerardus Cornelius Petrus de. 1853. Dissertatio physiologico-medica de actione atropae belladonnae in iridem. Utrecht: P. W. van de Weyer.


On hearing of CD’s work with Drosera, tells of his experiment showing extreme sensitivity of the iris of a dog’s eye to atropine. [See Insectivorous plants, p. 173.]

Letter details

Letter no.
Frans Cornelis (Franciscus Cornelius) Donders
Charles Robert Darwin
Sent from
Source of text
DAR 58.1: 79–80
Physical description
4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 9545,” accessed on 25 January 2022,

Also published in The Correspondence of Charles Darwin, vol. 22