My dear and most honoured Sir,

It is a long time, since I promised you to write you soon again.¹ But knowing your extreme kindness and proud in your confidence, I will not try to apologise, which might not be very difficult.

Your question is this: “As you say that the eyeball advances in adaptation for near vision for close objects, would the eyeball have to be pushed backwards in adaptation for distant objects”?²

In regard to this question, the first remark is, that we really do not actively accommodate for distant objects: the accommodation for distant objects is the passive condition of the eye, the relaxed. Therefore, if we start from a point, which is situated between the nearest and the most distant, indeed, the eyeball advances, if we accommodate for the nearest, and moves backwards, if we approach to the most distant.— This, however, is included in my first result, that the eyeball advances more and more in the accommodation from the most distant to the nearest point.³

Now, I have examined also the eye of a person, “who is lost in meditation”, and I have been observed in this condition by my son-in-law, Prof. Th. W. Engelmann.⁴ The result is, that, in such a state of meditation, the eyes are not fixed on a distant point, but, without real fixation, the lines of vision often become even divergent. This divergence attains in my eyes easily 1^o.87, in maximo, 2^o,—if the head is in a vertical position and the plane of fixation beeing horizontal. It increases in maximo at 6^o8, if we look very much upwards. Now in meditation very often the head is inclined forwards and the lines of vision are nearly horizontal, and in these cases the divergence easily attains 3^o or 3^o5.— I think this divergence is rather characteristic. It is proved by the crossed dopple images of a distant object, and it offered no difficulty to determine the angular distance of these images.— I think, this divergence is quite passive, and consists in a full loss of contraction, in a fuller relaxation of the internal muscles.

Now we found, that looking at a distance, without divergence, the divergence following by meditation is not attended with any farther mouvement of the eyeball, provided there be no mouvement in the eyelids. But as soon as the under eyelid ascends a little (which sometimes might happen involuntarily, the eyeball really is moved a little backwards. The fact is quite sure. Only, in the same the upper eyelid also uses to descend a little. Now, this also confirms the general rule, that, when the two eyelids approach each other the eyeball is moved backwards; and forwards as soon as the aperture between the eyelids increases. It is, indeed, very curious to see, how regularly these mouvements of the eyelids are attended with mouvements of the eyeball. It gives the impression, that the eyeball, always beeing pushed forwards by a continual pressure from behind, is pushed backwards by two valves by and by, whilst these approach each other. But I must avow, not to be able to determine with accuracy, how far these mouvements of the eyeball are to be considered as an associated contraction, how far as a pressure (by clothing), how far as a result of the entrance of the substance of the eyelids into the orbit (by opening), by which the eyeball could be pushed forwards. Therefore, I can not affirm your question: can the wrinkling of the lower eyelids (in meditation) act in pushing back the eyeball? For the same happens without wrinkling and is not distinctly increased by wrinkling and—this mouvement of the lower eyelid seems to be regularly accompanied by some, though less distinct, mouvement of the upper eyelid.

On the pupil of parrots, I did write you before, and suppose, that you will have had the opportunity of convincing yourself, how far the mouvements of the eye are engaged in the mouvements of the pupil.⁵ I hope, you have received my book and perhaps found any leisure to read the little account on the innervation of the iris.⁶

“When the heart beats hard and quick, and the head becomes somewhat congested, does the pupil contract?”⁷

I suppose that the conditions in the brain may be very different in that case. As long as there is irritation, very often the pupil is contracted, afterwards dilated. An analysis of the different conditions of the different nerves, acting on the pupil would be necessary. I think, the condition is too complicated for allowing a general solution.

In incipient faintness, I very often saw dilatation of the pupil; in utter prostration, I had no opportunity. I suppose this condition might be also complicated and rather different.

I would be very happy, if my imperfect indications could have any value for your work. Now, I can thank you for all the learning I took from your splendid book. I would be most happy to hear you on same questions. Perhaps next year, I might come again in England, and I hope, you will allow me, to visit you again.⁸

With the highest regards | Yours very sincerely Donders