But a better method is to employ the stereoscope, an instrument invented for the purpose by Mr. Wheatstone, the essential parts of which are two plain mirrors inclined with their backs towards each other at an angle of 90°. The two pictures A and B are placed in the same horizontal line, and parallel to each other at the sides of these mirrors, and at equal distances from them. The observer then placing his eyes as near as possible to the two mirrors, their angle coinciding with the middle line of his forehead and face, sees the solid body represented by the perspective drawings standing forward in relief, provided the two drawings are so situated that their images reflected by the mirrors coincide with the lines of the convergent optic axes. Instead of the original reflecting stereoscope of Mr. Wheatstone, an instrument of the refracting form introduced by Sir David Brewster, and now so common, may be used in the experiment. "%Then similar images, differing to a certain extent in magnitude, are presented, by means of the stereoscope, to corresponding parts of the two retinae, a single object, intermediate in size between the two monocular pictures, is seen. Were it net for this, objects would be seen single only when the optic axes converge directly forwards; that is to say, when the object is equally distant from the two eyes ; for it is only then that the images on the retinae can be of equal size, the size of the image being dependent on the angle under which the object is seen, and this being'less as the object is more distant. As our conviction then of the solidity and projection in relief of bodies depends upon a different per spectRe image of them being presented to each retina, and as this can only take place when the axes of the eyes are made to converge to them, it follows that when objects are at such a distance that in regarding them the optic axes are parallel, their' images on the retinae will be exactly similar, and the idea conveyed to the mind will be the same as if they were seen with one eye only. Hence, when two perfectly similar pictures of an object are viewed in the stereoscope, although they coalesce, they appear but as painted on a flat surface. With a knowledge of these facts, it becomes easy to explain why the artist is unable to give a faithful representation of any near solid object, that is, to produce a painting which shall not be distinguished in the mind from the object itself. When the painting and the object are seen with both eyes, in the case of the painting two similar pictures are projected on the retinas ; in the case of the solid object the pictures are dissimilar; there is therefore an essential difference between the impressions on the organ of sensation in the two cases, and consequently between the perceptions formed in the mind; the painting therefore cannot be confounded with the solid object. As our belief then in the solidity of a near object is owing to our taking cognisance of the impressions on both retinae, it is interesting to inquire whether any other kind of information is imparted to us by the possession of two eyes, which we should not obtain by one only. It is well known that if we close one eye, and attempt to judge of distances with the eye that remains open, our conjectures are wide of the mark, and the rationale of this has been explained by Le Cat, in his Tmit6 des Sensations,' in the following words :—" The concurrence of the optic axes, and the length of the angle they form, are the fundamental principles for esti mating the distance of objects : hence it is that when we look with one eye only, we are imablo to distinguish distances, and cannot place the end of the finger directly upon an object indicated to us, though it be very near, for the finger hides the object, and appears to correspond to it as exactly when it is at the distance of a foot, as if it were only a line removed from it. But if our other eye be open, it will ace the finger and the object from the side, and will therefore discover a con siderable interval between them if they are a foot distant from each other, but only a very small interval if they are very near ; and thus we are enabled to place our finger with certainty upon the desired object." The convergence of the optic axes which takes place when we regard objects within a short distance of us, is supposed by many to assist us in our judgment of the magnitude of bodies, and if this is admitted, it is another proof of the variety and extent of information con veyed to the mind by the possession of two eyes, which a single eye could only have afforded with the aid of the movements of the head and of the sense of touch. The confusion of vision and the indetermination of judgment which follow the lees of an eye, often continue for many months, and strikingly illustrate the truth of the foregoing remarks. Our estimation of the distance and size of remote objects is purely a matter of experience ; an object appears distant in proportion to its indistinctness of colour and outline, to the number of intermediate bodies seen between it and the observer, and to its appearing relatively smaller than these. We judge of the magnitude of objects by a calculation founded on their apparent size and pro bable distance : hence we are liable to continual mistakes on these points. An Englishman in the clear atmosphere of Italy supposes distant objects to be nta?rer to him than they are. A mountain which we see at a distance for the first time appears ?enerally much less than it really is, and we think it near us when it is very far away. From these remarks it is evident that the mind is constantly co-operating in the acts of vision, so that it becomes difficult to say what belongs to mere sensation, and what to the influence of the mind : that the latter must take an active part in the conceptions of vision, is evident from the great difference in the extent of the actual and the mental field of vision. The one is dependent on the extent of the retina ; the other has no determinate limits : in one, all objects are of equal magnitude that are seen under the same angle, and therefore produce an image of the same size upon the retina; in the other, the images of these objects though viewed under the same angle, are of various sizes and placed at very different distances.
It is scarcely necessary to say much in reference to the movement of bodies: we judge of their motion partly from the movement of their images over the surface of the retina, and partly from the movement of our eyes following them. If the image upon the retina moves while our eyes and body are at rest, we conclude that the object is changing its relative position with regard to ourselves. In such a case the movement of the object may be apparent only, as when we are fixed upon s body which is in motion, such as a ship. On the other hand, the image may remain fixed on the same spot of the retina, while our eyes follow the moving body ; we then judge of its motion by the sensations in the muscles which move the eyes. If the image moves only in correspondence with the actions of the muscles, as in reading, we infer that the object is stationary. The sensations of rotatory movements of objects, produced by turning the body on its axis, are quite independent of any impressions on the retina, and their con sideration is therefore foreign to the subject we are treating of. The apparent movement of objects after looking at these really moving, may arise from the successive disappearance of spectra left by the moving bodies. From the fact that artificial excitement of the retina,
either by pressure, electricity, or any other cause, gives rise to the perception of colour as well as light, we infer that the retina is the seat of these sensations. The colour of luminous bodies depends upon the quality of the light they emit ; the colour of bodies that are not luminous is due to the light which falls upon them, and is reflected by them towards our eyes. When a body absorbs all the rays of light which fall upon it, its colour is black; when it reflects them all, it is white ; and when it absorbs some and reflects others, it is coloured. [ABsolusriosid The question has often been raised, why is it that we see objects erect, while their images on the retina are inverted? According to most physiologists, it is by virtue of a certaiu property of the retina by which each point of an object is seen in the direction of a line perpendicular to its surface; now since this surface is concave, the rays proceeding from an object which fall on the lower part of its concavity will incline upwards, while those which impinge on its upper part will incline downwards ; and thus the object presented to the mind will be the reverse of that which is depicted on the retina. Many physiologists reject this theory, on the ground that it involves an im possibility, since each point of the image is not formed by rays having one determinate direction, but by an entire cone of rays; they affirm moreover that vision can consist only in the perception of the state of the retina itself, and not of anything lying in front of it in the external world. They argue further, that no explanation of erect vision is required, as long as all things equally, and not some objects only, appeared to the eye inverted ; for nothing can be inverted where nothing is erect, each idea existing only in antithesis to the other. A question not less agitated than the one we have just discussed, is that of single vision with both eyes. We shall not inquire into the merits of the various theories that have been invented in order to account for this phenomenon ; but shall merely advert to the principal conditions which are essential to single vision, in order that we may explain under what circumstances double vision results.
If two fingers are held up before the eyes, one in front of the other, and vision is directed to the more distant, the nearer will appear dabble, while if the nearer one is regarded more particularly, so as to appear single, the more distant will be seen double, and one of the double images in each case will be found to belong to one eye, and the other to the other eye. This phenomenon has given rise to the hypothesis that there are certain corresponding or identical points on the two retina, and that when these are affected simultaneously, single vision results ; while if the image of an object falls on parts which are not identical, it is seen double. A knowledge of these facts is obtained in the following manner :—If in a dark room with our eyes closed we make pressure with the finger upon any part of the ball of the eye, so as to affect the retina, a luminous circle will be seen in the field of vision at the opposite side to that on which the pressure is made. If we press on both eyes simultaneously, one luminous ring is seen when " identical " parts are pressed on, and two rings when " non-identical " parts receive the pressure. By this means it has been ascertained that the upper and lower portions of the two retinas are identical with each other, and that the outer lateral portion of one eyo is identical with the inner portion of the other, and so of the intermediate parts. Now whenever the axes of our eyes converge to an object, its image falls on corresponding portions of the two retina), and is seen single ; when we regard it without making our optic axes meet in it, as in the experiment of holding up two fingers, non-identical parts of the retinre are affected, and it is seen double. To illustrate this, let a be a point towards which the axes of the eyes are directed, and b an object more distant from the eyes. An image of a will fall upon identical points of the two retinae, namely, upon the central points 5, 5; a will consequently be seen single. The image of b will fall in the left eye at 6, and in the right eye at 4. The points 4 and 6 of the two eyes being non-identical (since the identical parts are marked with corresponding figures), b will be seen double; and the distance between the two images of b, in pro portion to the extent of the whole field of vision, will be the same as that between 4 and 6, in comparison with the distance between 1 and 10 in each retina. The centre of the retina furnishes the most distinct vision, therefore double images, which generally fall on the lateral parts, are indistinct. The position of double images depends upon the point at which the axes of the eyes decussate ; if in front of the object, the left hand image belongs to the left eye, and the right hand image to the right eye ; while if the axes converge to a point beyond it, the converse of this is observed. That objects will not in general be seen single, unless their images fall on corresponding portions of the two retinae, is further confirmed by the phenomena which are observed in strabismus, and by the experiment of displacing the axis of one eye by pressing on it with the finger ; but that exactly identical points of the Minx must be affected by similar points of the two images, is stall ciently refuted by Mr. Wheatstone's discovery of different perspective projections being presented to each eye. Mr. Wheatstone has also shown that under some circumstances similar pictures falling on cor responding points of the retina) may appear double and in different places. If to one eye we present in the stereoscope a vertical line n, to the other eye a line inclined some degrees from the perpendicular, we shall see a line the extremities of which appear at different distances before the eyes. If now we draw a faint vertical line, intersecting the inclined line at its centre, as at A, and present the two drawings to the eyes as before; the two strong lines will still coincide, and the resultant perspective line will occupy the same place, while the faint line, though it occupies the same part of the retina as the vertical line B, appears in a different place, namely, at the intersection of the planes of visual direction of the two eyes. In quadrupeds the relation between the identical and non-identical parts of the retinae cannot be the same as in man, for the axes of their eyes generally diverge and cannot be made to meet in one point of an object. :Midler therefore supposes that there are parts of their retina) which are identical, and parts which are not identical, which have no corresponding parts in the other eye, and the relation of the two retiun to each other in the field of vision may be represented as below. Although the theory of corresponding points is the most perfect that has yet been offered in explanation of the phenomena of single and double vision, yet the facts which have been advanced against it by Mr. 'Wheatstone are sufficient to show that it cannot be adopted without some limitation. Were it even not liable to these objections, it would still only express the conditions required for single and double vision, and would leave unexplained the cause of two impressions giving rise to one sensation.