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Dutour (1763)

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As found by Dutour (1763) rivalry occurs .... (O'Shea, 1999).

...rivalry was found (Dutour, 1763; translated by O'Shea, 1999).

How to reference the original work and this translation

Dutour, E.-F. (1763). Addition au Mémoire intitulé, Discussion d'une question d'Optique, imprimé dans le troisième Volume des Mémoires des Savan[t]s Étrangers, pages 514 & suivantes. [Addition to the Memoir entitled, Discussion on a question of Optics printed in the third Volume of Memoirs of Foreign Scientists, pages 514 and following]. Academie des Sciences. Mémoires de Mathématique et Physique Préséntes par Divers Savants, 4, 499-511.
O'Shea, R. P. (1999). Translation of Dutour (1763). Retrieved from

Addition to the Memoir entitled, Discussion on a question of Optics printed in the third Volume of Memoirs of Foreign Scientists, pages 514 and following

26. This continues experiments which I reported in the first Memoir. There it appeared conclusive that at the moment the mind perceives the image on an object projected on the retina of one of the eyes, the impression of the image on the corresponding portion of the other is ineffective and useless. However the experiments suffer the disadvantage of requiring crossing of the eyes such that fatigue results; it is very difficult to hold them in such a forced state for any length of time, even briefly. This observation was made to me while I had the honour of reading my Memoir to the Academy; also could I devise new experiments free from this constraint on the eyes, which makes the earlier ones difficult to effect. Below are two substitute experiments, which are much more comfortable to execute.

27. I used two glass disks, of an inch in diameter, one coloured blue, the other yellow. When combined they would make objects I looked at appear to have a range of shades of green. I attached each to a tube of cardboard, blackened on the interior, three or four inches long with a diameter equal to that of the glass disk. I looked through one tube with my right eye and the other with my left, at a piece of white paper held four to five feet away. The paper appeared to me sometimes blue, sometimes yellow, but never green, as ought to have occurred if the impressions received at the same time on corresponding portions of the two retinas had together influenced my perception of the colour of the sheet of paper.

28. With regard to the above changes between blue and yellow, it can readily be seen that they would arise from alterations in the configuration of my eyes during the course of the experiment. That is, at certain moments, light rays leaving each point of the object would reunite closer to the retina for the blue in one eye than for the yellow in the other, and at other moments it would be the opposite. Those of blue or yellow rays which projected the more distinct image would affect the mind by choice; they would give their colour to the object.

29. If one applies a prism held vertically before one of the eyes, so only refracted rays of light are passed to that eye, and with the other eye open, it is certain that different objects will be projected on corresponding portions of the two retinas. If such images could both be simultaneously perceived, a confused picture would be seen. It would be similar to the results of successively printing two totally different drawings, one upon the other, on a single sheet of paper. But this is not what was evident to me in the test I made: sometimes I would see only objects projected in the bare eye, sometimes only those in the eye covered by the prism, and sometimes the objects projected in one would seem to me to intermingle with the objects projected in the other. What was perceived always seemed to depend on the amount of light any object would reflect or to the actual configuration of my eyes. From one moment to the next I was able, by moving my cornea or crystalline capsule, to have in the scene new objects which would replace the others I had formerly seen there. Each visible object would always be distinct in all parts, clearly outlined, coloured and complete.

Instead of a prism, a little mirror may be positioned sufficiently close to one of the eyes for it to be struck only by reflected rays of light, which would not reach the other eye. With both eyes open, the same phenomena of the previous procedure will be experienced, the procedures being perfectly analogous.

30. The case where the the scene consists of objects simultaneously projected in the right and left eyes, and is, as it were, formed of combined patches, may not be regarded as a disorder produced by the co-operation of two impressions received on corresponding retinal areas. This effect arises only when certain portions of one of the retinas are effectively excited, and others not. Areas on the other retina, corresponding to these latter portions, receive impressions that pass up to the mind, by causes similar to those I stated in number 28. That is, images of objects are not all equally distinct in the two eyes. For example, sometimes images are more distinct in the right eye than their corresponding images are in the left, whereas the opposite holds for the remainder of the images in the right eye. Now, it always follows that of two corresponding images, the more distinct erases the impression of that which is less. In this way, along with the unequal amount of light which objects represented in one or the other eye are able to reflect, the two contribute, each for its part, to the production of the scene. All here is analogous to experiments numbers 8 and 23.

31. We may now deal with the observation that when the scene is thus formed of related pieces, certain objects are sometimes less bright and a little less distinct than if the eye where they are projected were the only one opened. This may at first glance be identified as the disorder that the projection of different objects on corresponding portions of the two retinas would cause in vision. If this were the real cause, however, why would not an equal lack of clarity and vivacity be similarly noticed in the picture, when it represents only single objects projected in the open eye? This is another observation by which it seems to me only natural to be struck. Consider the following: objects at different distances will be projected within each eye, and the prism will cause differences between the eyes. In order for equally distinct vision in each eye, rays emanating from each point of the respective objects would have to reunite equally on, or at equal distances from, the retinas. This, I maintain, would require such different configurations of the two eyes, for example, in the degree of convexity of the two corneas, that it would be unlikely that they could be achieved. It therefore follows that for all the objects which enter in the composition of the scene to be equally distinct is almost impossible. Now, these circumstances are without doubt those which obtain in the case under question here.

32. In the examples reported in my first Memoir, perception resulting from portions of images simultaneously received in the two eyes was comprised of all of one part of each image. Specifically, what was to the right of one optical axis, and all the opposite part of the image projected to the left of the optical axis in the other eye, were perceived as fitting together. Moreover, it is evident from the experiment with the prism or the mirror, number 29, that any part of one eye's image, can be as though inserted in the scene in one or more detached places, and isolated among those images occupying noncorresponding portions of the other eye. The following observations will point this out still more clearly.

33. When one eye, the left for example, is covered by a red coloured glass, and one faces a large window, its panes will appear red, if one looks only with the left eye. Now, close the left and open the right, and hold a raised finger between the right eye and the window, so that it hides an intermediate and vertical set of window panes from the right eye. Without moving the finger, when both eyes are opened, the panes which the finger hides from the right eye will appear red and can exist only in the left eye, and the noncoloured panes can exist only in the right eye. In these circumstances a set of red coloured panes will be seen between sets of noncoloured panes. It follows that perception from the image of the right eye is divided in two and separated by that from the image of the left eye.

34. And we are now going to see that the impressions of images received in the two eyes can interrupt each other infinitely in perception. Cover the left eye with a prism and arrange things so that any quantity whatsoever of small disks of white paper are visible on a dark background. With the uncovered eye view a wainscotting painted uniformly of a darker colour than the paper disks, but brighter than their background. When both eyes are open, the paper disks will seem to be applied on the wainscotting, that is, as many portions of the image of the left eye will appear sprinkled among the portions of the image of the right eye. The same experiment may be executed even more easily and with equal success, by substituting a small mirror for the prism over the left eye, as in that of number 29.

35. The conclusions I have drawn from these two observations, may be used to explain why a sheet of white paper can seem brighter when viewed with two eyes, than when viewed with only one.

The surface of the sheet of paper has enough perceptible irregularities that one can consider it as formed of an infinity of little hillocks and pits. All the points would not be visible to either eye. There are surely some of these points which are not projected on one retina but are projected on the other. On each retina there will be small portions where no light falls which have corresponding areas of the other retina which are struck by the rays leaving points of the paper not visible to the first eye. That way, one must indeed discover more points of the surface of the paper, and the surface must appear more brilliant when both eyes are fixed upon it than when only one is used. It follows that while the totality of the mind's perception can be more marked in the first case, this still would be produced only by the impressions received on noncorresponding portions of the retina which are intermingled in the scene, in the way I stated above. It does not at all require that impressions on corresponding areas be perceptible at the same time.

36. Here is yet another fact which lends itself to the same explanation. When I covered one eye with a coloured glass, and viewed a sheet of paper with both eyes open, I observed that it did not appear as white as when I viewed it only with the uncovered eye. The difference was perceptible only when I would alternatively look with the bare eye then abruptly open both. If the two eyes were used first, the paper would be judged sufficiently white to give no hint of a colour change. Nevertheless, it is still true that certain rays transmitted by the coloured glass can influence perception, despite being principally and almost totally determined by light reaching the uncovered eye. This may be made consistent with the observations in the preceding section as follow: Light passing through the coloured glass would influence perception when it fell on portions of the retina corresponding to areas in the uncovered eye which received no light. The sheet of paper would derive its perceived colour from these coloured rays.

37. I will add that have not commented on similar differences when each eye views through differently-coloured glasses, such as blue and yellow. I would alternatively look at the paper at first with one or the other eye, then with both eyes at once. The paper appeared to remain either yellow or blue, without the least reduction in clarity or saturation. I believe this may be attributed to the fact that an easily-perceivable deterioration of a white object would be be far less noticeable on an object dyed of one or the other colour, by the same reason that white material is sooner dulled than blue or green material.

38. The explanations I have given of the facts in numbers 31, 35, 36, provide solutions to any difficulties for my view which might be proposed to establish the opposite: that corresponding parts of two images can effectively influence vision. I have yet to present another objection, which turns on the fact that eyesight is more quickly fatigued when viewing an object with only one eye than with both open. The effort necessary to keep one eye closed and the other open is undoubtedly a component, however it may be removed if all light were prevented from reaching one of the open eyes by covering it with the hand or some other opaque object. In that case, one would have to seek other causes for the effort. To discover these, notice first that one can distinguish two kinds of vision: one exact and, so to say, active, and the other vague and, so to say, passive. Active vision is that which takes place when an object is viewed so as to perceive clearly its form and components, as for example, when one reads. This type of vision always requires, or almost always, that certain portions of the eye take configurations different to when at rest: examples include whether the pupil contracts or dilates, whether the cornea would become more convex, or whether the anterior portion of the crystalline capsule becomes less; anything which cannot be achieved without some effort. Passive vision occurs when one does not pay close attention to any particular object, and with the eyes in their rest positions, one indifferently receives sharp or blurred projected images, without wishing or intending to make the least effort to discern the form and diverse parts of objects. In the last case, I would willingly believe that there is no more fatigue when one eye is open than when both are open. However, in the case of active vision, it would seem to me that the effort necessary to change any part of the eye from its resting configuration, cannot fail to fatigue eventually. There would undoubtedly be more fatigue if it were necessary to alter both eyes at the same time, than if only one needed to be changed. That is, more effort would be required if both eyes had to be used together to see an object distinctly, than if one were necessary. Conversely, it follows that when we have both eyes open, we need only one for distinct vision, the other being at rest. This allows us to explain why sight is more quickly fatigued if one has only a single eye open on the object which one wants to see distinctly than if both are open. Now, when both eyes are open they have use some effort to change their parts for clear vision of an object. If only one changes, the other in which blurred or less clear images are projected, would retain its natural configuration, and would not be fatigued in the least. The first will not fail to tire after some period of time, but will be relieved when it is allowed to resume its natural configuration, at the same time that the other eye will be burdened with the exercise of vision. This one will adjust its cornea and lens to achieve clear vision of the object which was only confused for it the moment before. The two eyes transfer their functions between themselves, working in shifts and replacing each other in rotation; neither of the two would tire, because each acts and rests at intervals. When one is closed or covered, however, the work of vision depends totally on the other. It being fixed without respite on objects one wanted to see distinctly, cannot fail to be worn out eventually by the unaccustomedly continued effort it must use to hold certain of its parts in a forced disposition. The case I am coming to advance is that when both eyes view an object, they would ordinarily be used only alternately for the work of distinct vision. We have another clear indication of this in the results of tests reported in numbers 2 and 27. In these my eyes were simultaneously fixed on a blue object for one and a yellow object for the other. I saw yellow and blue in alternation.
We may now deal with a second cause for the quicker fatigue of distinct vision if only one eye is open than if both are: namely, the impressions of light on the retina which wound it, if they are too bright or too continuous. While only one eye is open and applied to distinct vision, it endures without interruption the impressions of bundles of rays leaving each point of the object. That eye bears the most intense light since in this case these bundles of rays have their point of convergence on the retina or very near the retina. When both eyes are open, however, they exercise the functions of vision only in turn. Each eye rests itself by intervals, neither one remaining exposed very long at a time to overly vivid luminous impressions. There is only the retina of the eye which is currently on duty that endures the total intensity of bundles of rays leaving each point of the object. By the configuration of the other inactive eye, the focus of the bundles of rays falls well in front of or behind the retina. In this way, the image of the object occupies a larger area, and its light is distributed over a larger number of parts. Hence the retina is less unsettled, so much so that that the ray of dissipation, according to the doctrine of M. Jurin, approaches equality with, or even surpasses the ray of the real image.

39. Note that while the alternate action of the two eyes is natural and usual, it is possible to force them into a state in which objects are sufficiently distinct to both eyes simultaneously. However, even in this case of equally clear images, only noncorresponding parts effect perception, as I have explained above. Besides, is this not a state of constraint?

40. Indeed, the considerations announced in number 38 are leading us to suspect strongly that there must be one natural cause, by virtue of which only unequal images are ever projected on corresponding parts of the retinas, with the result that one image would be erased by the other dominant image. I have advanced the case that the eyes need to relieve each other alternately and view distinctly only at intervals, so as not to be worn out by the exercise of vision. In the light of this necessity of alternative exercise and intermission, surely the Creator is entailed. He has had to use the proper means to prevent both eyes from being fatigued at the same time by holding one in inaction while the other acts, so that distinct vision were not less than continuous. It would surely not behoove us to decide on the wisdom of His choice; but there seems to be a simple and convenient way for the design. That is, the two eyes He has given to man do not fuse absolutely equally, but are arranged to achieve the opposite. For example, in one eye, the natural curvature of its cornea and lens make the apices of pyramids of rays from a particular object end precisely on its retina. Slight differences in the convexity of the cornea or lens of the other eye, will cause similar rays to reunite somewhat beyond or short of its retina. Hence, the object would not be as distinctly visible with that eye, unless some effort were made to augment or diminish the convergence of light rays by changing the curvature of the cornea or the lens. This supposition should appear all the more plausible because of differences in the eyes of any individual; to require that the eyes be exactly the same seems excessive, in view of the claim by famous Philosophers that Nature does not provide any examples of such similarity. Once accepted, it is conceivable that as soon as an object is projected distinctly in one of the eyes, its image must naturally be less clear in the other. This is because once an object is sufficiently clear, by virtue of the impression of the image received in the first eye, an observer will gladly spare himself a constraint and a surplus effort, to make its image as distinct in the second. In ordinary cases, therefore, only a single eye is used at one time for the exercise of distinct vision. In that moment the other rests itself and will negligently perform its function, except for it to take its turn to see in the moments of respite required by the first.

41. This respective nonconformity of the two eyes can thus be supposed to be universal. If there is any individual who has the two eyes naturally conformed the same, would this be a peculiar exception? It is an already recognized fact that in the majority of men the two eyes do not have clear vision within the same limits [Buffon, 1743]. Moreover, in ordinary cases and where vision is, as it were, left to itself, it readily follows from my proposal that one of the impressions made by unequal retinal images preferably affects the mind and that the mind pays no attention to the other. It also explains why in ordinary cases, all the distinct portion of the scene or the perception of the mind is derived from only one set of the two images. In cases where vision of an object is constrained to the simultaneous use of both eyes, as it is necessary for distinct vision only that the rays of light from the object reunite precisely on one retina, it also readily follows that one never pushes the effort to the point that those rays are equally reunited on both retinas or at equal distances from them. Thus one of the two images remains more clearly defined than the other, and thereby takes the advantage of affecting the mind by choice. One can even well imagine that the two images differ sufficiently in yet other respects, so that their impressions would never, or almost never, have the equality to make them both effective at the same time.
I have used the above principle to explain why objects which have double images in our eyes appear single. It still remains for me to extend the principle to several phenomena that present us strabismus, but this will be the material of a special Memoir.

ERRATA for the Memoirs of M. DU TOUR, printed in the third Volume of Foreign Scientists.

Page 234, line 25, instead of montées, read moulées.
236, 7, instead of poids, read point.
237, 7, instead of ce qui, read & qui.
243, 24, the the end of the line, delete the word ce.
517, 22, instead of cordon, read carton.
526, 33, instead of marquoient, read masquoient.

On the subject of the Plate accompanying the Memoir on Optics, it is necessary to point out that the Printer mistakenly used a mirror to engrave figures 3, 4, 5, 6, 7, 10 and 11. Hence what should be on the right is on the left, and vice versa. The reader is invited to correct this, by reading right in places where left is referred to in these figures, and vice versa. Alternatively, and this amounts to the same thing, invert the plate bearing the figures.

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