My dear Prof: Donders,

Your very kind letter has interested me greatly, & it is quite a relief to me to find that such minute doses of atropine act on the muscles & the iris.¹ As you remark the eye of a dog is a large object, & the quantity of atropine which acts on each fibre must be as small, or smaller than the quantity of phosphate of Ammonia which acts on a single gland of Drosera. I wish that the quantity of the solution, & therefore of the salt of atropine, which was applied to the eye had been measured; but this perhaps w^d. have been useless, as it might have been impossible to ascertain how much was absorbed. Pray thank Pro:. Engelmann for me; I should very much like to see his work, which I c^d. return after consulting it, & if he will mark the passages bearing on minute quantities, it would be a great aid, for I read German with much difficulty.²

If D^r. De Ruyter enters into details on the quantity applied I should also like to see his book; but I fear I am very troublesome.³ I shall not return to my M.S. chapter on this subject until one or two months have elapsed.⁴

With cordial thanks, I remain yours very sincerely, | Charles Darwin

P.S. My son George has just returned worse I fear than when he started.⁵