My dear Mister Darwin,

I am very much honoured by your last letter, which makes some other enquiries from me.¹ Upon these I can give you only a provisional answer.

The idea that in meditation the eyeball might sink a little into the orbit, is not at all fanciful: I hope to be able to tell you how it is, within a few weeks.

On the influence of different passions on the pupil I cannot say you much for the moment.² I will try to know some thing about and if I have some success, will give you an account of it. The chief point why I write provisonnally to you, is this: that it might be possible, that you are not aware of the influence of accommodation and of the associating motions of the eyeball (convergence) on the diameter of the pupil.³ I think, this is of some importance. I often saw that the pupil of parrots contract and dilate, independently of the amount of light. But I suppose, that these movements were combined with movements of the eyeball, the contraction coinciding with convergence. Perhaps in parrots exists some accommodation even without movement of the eye. I believe, H. Mueller demonstrated, that they have two points of exact (direct) vision on every eye, one for uniocular, another for binocular vision; and if they use that for uniocular vision, there is no reason, why birds could not accommodate for short distances, without moving the eyes.⁴ In man, that is not commonly the case; but after some exercise, I succeeded to contract my pupils, by the effort of accommodating for a near point, without any motion of the eye (of whatever of the eyes). Also the convergence, without change of accommodation, is combined with contraction of the pupil: this I could demonstrate by putting prismatical glasses before the eyes, the angles turned inwards, which require more convergence for binocular vision of the same points, and meanwhile an increase of accommodation can be avoided.— Perhaps the whole doctrine of the movements of the iris is in some respect important for your studies on physiognomy. You may find that in every good physiological book. I hope however that you will allow me to send to you a copy of my book on the anomalies of accommodation, etc., which contains a short exposition of the subject p. 572–575, followed by that of the ciliary system and its function.—⁵ In the same book you find (p. 197) a description of the entoptic method. It may be of some interest for you, not for the irregularities in the eye, but for the opportunity which it gives to see the changes of the diameter of the pupil in your own eye. As it is mentioned p. 573: A small opening in an opaque plate, held at about $\frac{1}{2}$ inch from the eye and turned towards the light, gives, in the vitreous humour a bundle of nearly parallel rays, of the size of the pupil, and is therefore seen as a round, illuminated disc, whose diameter increases and diminishes with that of the pupil. This method allows us to judge very accurately about motions of our pupils, which might occur, whilst we produce in (or suggest to ourselves different animi pathemata.⁶ The opening in the plate should not be too small for this purpose,—perhaps $\frac{1}{4}$ Millimeter;— an opening made with a needle in a piece of thick black paper will do very well.

If all this has no value at all for the sake of your researches, I implore your pardon for the trouble I give you by adressing this letter.

I ordered a copy of my book to be sent to you in England, where it is published.

I am obliged to go to Leipsick⁷ and Berlin for a couple of weeks. Therefore, I must pray you to give me some time more for entering, if possible, in further details into the questions, which you mention in your letter. That there is nothing, which I wish more than to remove some obstacles in your researches, as far as my special knowledge of the one or the other point may afford, I hope you are persuaded. Will you kindly excuse my bad English?—

Believe me respectfully and yours very sincerely | Donders.