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touched the outline of a distant hill in the one picture, but was "a full moon's-breadth" from it on the other. When these dissimilar pictures were united by the eyes, a landscape, certainly a very imperfect one, was seen in relief, composed of three distances.

Owing, no doubt, to the difficulty of procuring good binocular pictures, Mr. Elliot did not see that his contrivance would be very popular, and therefore carried it no farther. He had never heard of Mr. Wheatstone's stereoscope till he saw his paper on Vision reprinted in the Philosophical Magazine for March 1852, and having perused it, he was convinced not only that Mr. Wheatstone's theory of the instrument was incorrect, but that his claim to the discovery of the dissimilarity of the images in each eye had no foundation. He was, therefore, led to communicate to the same journal the fact of his having himself, thirteen years before, constructed and used a stereoscope, which was still in his possession. In making this claim, Mr. Elliot had no intention of depriving Mr. Wheatstone of the credit which was justly due to him; and as the claim has been publicly made, we have described the nature of it as a part of scientific history.

In Mr. Wheatstone's ingenious paper of 1838, the subject of binocular vision is treated at considerable length. He gives an account of the opinions of previous writers, referring repeatedly to the works of Aguilonius, Gassendi, and Baptista Porta, in the last of which the views of Galen are given and explained. In citing the passage which we have already quoted from Leonardo da Vinci, and inserting the figure which illustrates it, he maintains that Leonardo da Vinci was not aware "that the object (c in Fig. 2) presented a different appearance to each eye." "He failed" he adds, " to observe this, and no subsequent writer, to my knowledge, has supplied the omission. The projection of two obviously dissimilar pictures on the two retina, when a single object is viewed, while the optic axes converge, must therefore be regarded as a new fact in the theory of vision." Now, although Leonardo da Vinci does not state in so many words that he was aware of the dissimilarity of the two pictures, the fact is obvious in his own figure, and he was not led by his subject to state the fact at all. But even if the fact had not stared him in the face he must have known it from the Optics of Euclid and the writings of Galen, with which he could not fail to have been well acquainted. That the dissimilarity of the two pictures is not a new fact we have already placed beyond a doubt. The fact is expressed in words, and delineated in drawings, by Aguilonius and Baptista Porta. It was obviously known to Dr. Smith, Mr. Harris, Dr. Porterfield, and Mr. Elliot, before it was known to Mr. Wheatstone, and we cannot understand how he failed to observe it in works which he has so often quoted, and in which he professes to have searched for it.

This remarkable property of binocular vision being thus clearly established by preceding writers, and admitted by himself, as the cause of the vision of solidity or distance, Mr. Wheatstone, as Mr. Elliot had done before him, thought of an instrument for uniting the two dissimilar pictures optically, so as to produce the same result that is obtained by the convergence of the optical axes. Mr. Elliot thought of doing this by the eyes alone; but Mr. Wheatstone adopted a much better method of doing it by reflexion. He ni thaa fed to (waraa as iffnte, to be afterwards i^.-ribed, coasawhi; of two ptaae mirrors, placed at aa angle of iK>". to which s* gate tie nimr of Mereomxpe, aaniapatiag lb: ECiot both in the coaetracDon aad pubawrioii of iis rnveiaom, bat not ia the general conception afaanmaaoae.

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After onacubanr his a|wanalmt. Mr. TTheatstone proceeds to cnandcr (in a section entitled, - Binocular vision of objects of dhVremt magaimdeeO « what ejects wiH result :- ~ j-TSf-t-j sin:'..-;: :v ;_--.< ..r.-.-- '-> ;- r.scrir^de. to analogous parts cvf the retina," •* For this purpose." he savs. - two squares or circles, diferimi unn"»r.g/<r but not f rn-sTaganthr in sins, mar be drawn oa two separate pieces of paper, and placed in the stereoscope, so that the reflected image of each shall he equalhrdistant from theerebTwhich it at regarded. A viB Am i* an tkat astatitaravftmjr una? mtftwtmce fl>ejf Omkw nnw -.Yvnwa a tutoft rvnamnut aauati'ia." The fact of eoales>onee being supposed to be perfect, the author aext seeks to determine the difference between the length of two hnes which the eve can force into coalescence, or "the limits within which the single appearance saints." He, therefore, unites two images of

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Visaed with these emv; ..< assertions, he proceeds to

gho a sort of rule or law for abstaining the relative sine

mwqaal pictures which the eves can force into

5nu inequality, he concludes, must not exceed

'between the projections of the same object

» the most oblique position of the exes (U,

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both turned to the extreme right or the extreme left) ordinarily employed." Now, this rule, taken in the sense in which it is meant, is simply a truism. It merely states that the difference of the pictures which the eyes can make to coalesce is equal to the difference of the pictures -which the eyes do make to coalesce in their most oblique position; but though a truism it is not a truth, first, because no real coincidence ever can take place, and, secondly, because no apparent coincidence is effected when the difference of the picture is greater than what is above stated.

From these principles, which will afterwards be shewn to be erroneous, Mr. Wheatstone proceeds "to examine why two dissimilar pictures projected on the two retinae give rise to the perception of an object in relief," "I will not attempt," he says, "at present to give the complete solution of this question, which is far from being so easy as at first glance it may appear to be, and is, indeed, one of great complexity. I shall, in this case, merely consider the most obvious explanations which might be offered, and shew their insufficiency to explain the whole of the phenomena.

"It may be supposed that we see only one point of a field of view distinctly at the same instant, the one, namely, to which the optic axes are directed, while all other points are seen so indistinctly that the mind does not recognise them to be either single or double, and that the figure is appreciated by successively directing the point of convergence of the optic axes successively to a sufficient number of its points to enable us to judge accurately of its form.

"That there is a degree of indistinctness in those parts of the field of view to which the eyes are not immediately directed, and which increases with the distance from that point, cannot be doubted; and it is also true that the objects there obscurely seen are frequently doubled. In ordinary vision, it may be said, this indistinctness and duplicity are not attended to, because the eyes shifting continually from point to point, every part of the object is successively rendered distinct, and the perception of the object is not the consequence of a single glance, during which a small part of it only is seen distinctly, but is formed from a comparison of all the pictures successively seen, while the eyes were changing from one point of an object to another.

"All this is In Some Degree true, but were it entirely so no appearance of relief should present itself when the eyes remain intently fixed on one point of a binocular image in the stereoscope. But, in performing the experiment carefully, it will be found, provided the picture do not extend far beyond the centres of distinct vision, that the image is still seen single, and in relief, when in this condition."1

In this passage the author makes a distinction between ordinary binocular vision, and binocular vision through the stereoscope, whereas in reality there is none. The theory of both is exactly the same. The muscles of the two eyes unite the two dissimilar pictures, and exhibit the solid, in ordinary vision; whereas in stereoscopic vision the images are united by reflexion or refraction, the eyes in both cases obtaining the vision of different distances by rapid and successive convergences of the optical axes. Mr. Wheatstone notices the degree of indistinctness in the parts of the picture to which the eyes are not immediately directed ; but he does not notice the "confusion and incongruity" which

1 Phil. Tram., 1838. pp. 391, 392.

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