B7 SIGHT. 



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 retinse can be of eqtial 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- 

 spective 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 retinse 

 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 retinae ; 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 hi 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 retinse, 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 ' Trait<5 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 unable 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 see 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 

 bead and of the sense of touch. The confusion of vision and the 

 indetermination of judgment which follow the loss 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 nearer to him than they are. A mountain which 

 we see at a distance for the first time appears generally 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 hi 

 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 a body which is in motion, such as a snip. 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, 

 that the object is stationary. The sensations of rotatory 



SIGHT. 



588 



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 those 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. 

 [ABSORPTION.] 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 certain 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 morits 

 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 double, 

 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 hi 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 

 retinsc, 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 retime are identical with each 

 other, and that the outer lateral portion of one eye 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 retinso, 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 retinse 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 retime, namely, upon the central points 5, 5; a will consequently 

 be seen single. The image of ft 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 

 retinse, is further confirmed by the phenomena which are observed in 

 strabismus, and by the experiment of displacing the axis of one eye by 



