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retina upon which it falls, that is, the lower distinctly when we look away from it, that is, part of the image is seen above the middle when we look at a point distant from it part of the object, and as this is true of several degrees. This singular affection of every point of the image, the object will the retina will be better understood from appear erect from its inverted image. the following experiment :-At the distance
The best way of studying the phenomena of ten or twelve feet, look steadily at one of of vision, is to consider objects as made up two candles placed at the distance of about of points like a stippled engraving. Every a foot from each other. In a short time. point will have its image on the retina. the candle not looked at, but seen indirectly, The points in the upper part of the object will increase in size, and will be surrounded will have their image on the lower part of with & bright ring of yellow light, the light the retina, and vice versa, and those points of the candle itself having a pale blue colour. will, by the law of visible direction, be seen If, in the same manner, two objects not forming the upper part of the object. The very luminous, such as two pieces of paper, direction, therefore, of a line drawn from are placed upon a darkish ground, the one any point of an object through the centre of not looked at directly will vanish and reapthe pupil
, would be accurately the direction pear—a fertile source of illusion when faintof the sensation, if the centre of the pupil ly illuminated bodies are seen in a dark were the centre of curvature of the retina, night. When the light of an object is exor, what is the same thing, all the points of tremely faint, it will disappear and reappear the object would be seen in their true place. in irregular succession, even when the eye From this approximate coincidence of the is turned fully upon it, or when it is seen direction of the issuing ray, and the line in directly by the most sensitive part of the which the sensation is returned, objects retina. The eye is thrown into a state of seem to be fixed* when the eye ranges over painful agitation, and we attempt in vain to any object.
obtain sustained vision of the object. Although, on looking at any object, we We have already referred to the foramen obtain distinct vision of it, yet we do not centrale as the spot where vision is most see every part of it at the same instant distinct; but this is not its only optical proequally distinct
. When we wish to see perty. The writer of this article found that any point of it, for the purpose of examining it could be rendered visible, and its diameter that point, every other part of it is indistinct. measured in the following manner :-If when The reason of this is, that we direct the axis the eye has been for some time closed, and, of the eye to the point seen distinctly; and as it were, refreshed by protection from the image of that point is formed upon the light, we direct it to a faint white surface, foramen centrale, or central point of the re- such as that of a sheet of paper iluminated tina already mentioned, while the images of by a wax candle at the distance of ten or all other points are formed upon points of twelve feet, there will be seen on the the retina more or less distant from the paper a dark brown or reddish circular spot, foramen. This indistinctness does not arise shading off into the light of the paper. from any want of focal adjustment, but it is quickly disappears, and may be renewed by a property of the retina, arising, perhaps, again closing the eye for a few minutes. from the membrane being less pulpy, or This spot is, therefore, in the normal conhaving a finer surface at the foramen, or dition of the eye, less sensitive to light than elsewhere; or if the foramen is really an the other parts; that is, it takes longer time aperture, and the choroid coat behind it the to receive the impression of light from the seat of vision, the rays which fall upon the white paper. If the sensibility of the retina choroid at other parts must pass through has been previously reduced by a long exthe retina, and thus be made less distinct. posure to light, or by an exposure to much
But though the retina gives less distinct light, the circular spot is white, shading off vision on the parts of it away from the fora- into the light of the paper. In this abnormal men, it is much more sensitive to light in state of the retina, the foramen is more those parts; and hence it is remarkable quickly affected by light than the rest of the that, when we wish to see an object hardly retina. Hence it follows that, when the visible when we look at it directly, such, general retina is in the best state to receive for example, as one of the satellites of luminous impressions, it receives them more Saturn, or a very faint star, we see it most quickly than the foramen part of it, if it is
not an opening; and that when the general
retina is fatigued, or less capable of receiv* There is a slight motion arising from the refrac. ing luminous impressions, it receives them tion of the different humours. When we look through spectacles the motion is very great, owing more slowly than the foramen portion. The to the refraction of the lenses.
angular diameter of the circular spot is about
41°, which corresponds with a foramen about of the finger on the side next the nose, there the thirty-fifth part of an inch in diameter, will be produced "a circle of colours like as it has been found to be by anatomists. those in the feather of a peacock's tail." He These experiments are best made in the observes also," that if the eye and the finger morning when the eye has been long pro- remain quiet, these colours vanish in a second tected from the action of light, and in the of time, but if the finger be moved with a evening when it has been most fatigued. quivering motion, they appear again." In
Another part of the retina, much larger observing the effects of pressure, we have than the foramen, is wholly insensible to light found that a gentle pressure on the retina of ordinary intensity, and consequently all ob- produces a circular spot of light. By injects disappear when their images fall upon creasing the pressure this spot becomes dark, that part of it. This, however, is true only and is surrounded with a white ring of light, when we look with one eye, for in binocular shading off into darkness. When, in total vision the image in the other eye does not darkness, the retina is subjected to pressure, fall upon this insensible spot. The portion it gives out light; when it is exposed to of the retina thus insensible to ordinary light, compression increases its sensibility light, is at the entrance of the optic nerve, to light; when it is dilated, under exposure It is about the eighth of an inch in diameter, to light, it becomes insensible to luminous and is about 13from the foramen on the impressions. side next the nose. In order to observe This property of becoming luminous by this curious phenomenon, place two wafers compression shews itself on many occasions. at the distance of three or four inches from A sudden blow on the head or on the eye each other, and, shutting the left eye, look produces a bright flash of light. In sneezing, at the left-hand wafer with the right eye, and and in blowing air violently through the when its distance from the wafer is about nostrils, two patches of light appear above twelve inches, the right-hand wafer will the axis of each eye, and in front of it, while totally disappear, the spot which it occupied other two luminous spots united into one, being of the same colour as the ground upon appear about the point of the nose when the which the wafers are laid. If, when the eyes are directed to it. In turning the eyewafer is invisible, we open the left eye, it balls quickly by the action of their own will reappear; or, if we alter the distance muscles, the retina is pulled or pressed at of the eye, one side of the wafer will come the place of their attachment to the sclerotic into view—the innermost side when we in- coat, and a semicircle of light is distinctly crease the distance, and the outermost side seen opposite each eye, and towards the when we diminish it. The same results will nose. These semicircles, in certain states be obtained if we shut the right eye, and of the retina, are enlarged, and are somelook at the right-hand wafer with the left times expanded into complete circles of eye. In this case the left-hand wafer will light. In certain states of the stomach, acdisappear. But though*the base of the optic companied by headache, a faint blue light nerve, or the portion of the retina which it floats before the eyes in total darkness, passforms, is insensible to the light which falls ing across the field of view, and sometimes directly upon it, it is susceptible of receiving becoming green, yellow, and red. luminous impressions from the parts which In rubbing the eyes, specks or points of surround it. If the wafers are laid upon a light frequently appear, arising either from ground of any colour, the spot on which the the pressure being felt on some parts more wafer has disappeared will have the same than others, or from those parts being more colour as the ground on which it lies. But subject, from their nature, to the emission though light of ordinary intensity fails to of light. That this last cause is the more make an impression on this part of the retina, probable one, may be inferred from a very yet when candles are substituted for the curious phenomenon observed by the writer wafers the candle does not wholly disappear, of this article. If when the eye has been but leaves a sort of faint nebulous light, for some time exposed to the light of a gas which has no resemblance to the object from or any other flame, we suddenly extinguish which it proceeds.
it, there will be seen for an instant a great When we consider that the sensation of number, from fifteen to twenty, bright points light is produced by a material impression of light like stars, arranged in a circle, the on the retina, it might have been expected diameter of which subtends an angle of from that luminous effects would be produced by 700 to 90°. In this case there is no prespressure made upon the eyeball, and com- sure; and, therefore, the parts of the retina municated to the retina. Sir Isaac Newton, which emit these lights must have the proaccordingly, observed, that when we press perty of retaining luminous impressions the eyeball outwards, by applying the point longer than the other parts of the retina.
It is probable that the points of the retina, see the direction in which any object or i possessing this property, are those at its point of an object is situated, much more i margin, where it is in contact with the points distinctly than with two eyes. We see the where the cells of the vitreous humour have exact point where a near object strikes a : their origin behind the crystalline lens. more distant one, a thing which we cannot ! | The subject of binocular vision has recent- do with both eyes directed to it. When we ly acquired much interest from its connexion see the near object distinctly, the more diswith the stereoscope. That one eye is suf- tant one is doubled; so that in shooting ficient for the general purposes of life, is with a rifle or a musket, we cannot use both evident from the fact that many persons eyes. Some persons have the faculty of have spent a large portion of their life be shooting with both eyes open, but when they fore they discovered that they were blind do this they do not observe, or rather they in one eye, and also that those who have pay no attention to, the second image of the lost the sight of one eye by accident or dis- muzzle of their fowling-piece. In monocular ease, can perform with the other almost all vision, when two objects are brought into the operations which had been performed by the same line, they are not seen with the both. Two eyes, however, were required to same distinctness, but this is easily remegive symmetry to the human form, and no part died by looking at them through a small of the animal mechanism is more interesting aperture, which will show them equally disthan the contrivance by which the two work tinct. But though we cannot estimate disharmoniously together, and give new powers tances with any accuracy by one eye, there of observation and inquiry. In vision with are various means, called the criteria of disone eye the extent of the field of view is tance, by which we learn to form a pretty about 1500, while, with two eyes, it is about correct estimate of distances, but particular2000, but this depends upon the position of ly great distances. By these criteria, which the eyeball within its socket of bone, and are five in number, we are enabled in mono- ! upon the form and size of the cheek bones cular vision to appreciate approximately the and brow, An eye much sunk in the head, distance of objects. has a much narrower field than 1500, while 1. The interposition of numerous objects projecting eyes have a greater field than between the eye and the object whose dis2009. The left eye sees a larger field on tance we are appreciating. A distance at the left than the right eye, and the right eye sea appears much shorter than the same disa larger field on the right than the left eye, tance on land, marked with trees, and other so that each forms a dissimilar picture of objects; and for the same reason, the sun the external world, just as they do in view- and moon appear more distant when rising ing solid objects or scenes in nature. or setting on the horizon of a flat country,
It was long supposed that with two eyes than when in the zenith, or at great altiwe saw more brightly than with one, or that tudes. the luminosity of objects was doubled. 2. The variation in the apparent magniThis, however, is a great mistake. Dr. tude of known objects, such as man, animals, Jurin has proved by experiments which we trees, doors and windows of houses. If one have carefully repeated and found correct, of two men, placed at different distances that the brightness of objects seen with two from us, appears only half the size of the eyes, is only one-thirteenth part greater than other, we cannot be far wrong in believing when they are seen with one. We are dis- that the smallest in appearance is at twice posed to modify this result, and to maintain the distance of the other. It is possible that that in the normal condition of the eyes, the the one may be a dwarf, and the other of brightness of objects in monocular and bino- gigantic stature, in which case our judgment cular vision is exactly the same. In examining would be erroneous, but even in this case the state of the pupil, and measuring its other criteria might enable us to correct diameter in these two states of vision, we it. find it increased in such a degree as to admit 3. The degree of vivacity in the colours as much light when one eye is shut as when and tints of objects. both of them are open, so that in as far as 4. The degree of distinctness in the outthe mere brightness of objects is concerned, line and minute parts of objects. the loss of one eye is no disadvantage. 5. To these criteria we may add the sen
While two eyes are necessary for the pur-sation of muscular action, or rather effort, pose of symmetry in the human face, they by which we close the pupil in accommodawere required for other important purposes. ting the eye to near distances, and produce They enable us to see solid objects in a high- the accommodation. er relief, and all distances in nature more With all these means of estimating disperfectly than one eye. With one eye we tances, it is only by binocular vision, that
we have the power of seeing distance within|dle, and the remote distances in the landa limited range.
scape. Hence, when we shut one eye, we In binocular vision short distances are have not the power of discovering that the seen directly by the convergency of the op- picture is on a plane surface, and all its parts tic axes to the point observed. If the object equidistant from us, and consequently the is very near, it is very difficult to converge art with which the artist gives relief to the them without a great strain upon the eye. painting by light and shadow, or by the difWe feel this strain to be painful, and when ferent magnitude of objects of known size, or we remove the object to greater distances in by indistinctness of outline, and the other succession, the painful feeling is diminished. criteria of distance, exercises its whole effect This uneasiness arises both from the great in deceiving us into the belief that the pic muscular action necessary to bring the axes ture, portrait, or statue is in relief. of the eyeballs to converge upon an object This influence over our judgment is finely near us, and from the contraction of the pu- shown when we view with one eye photograpil and the simultaneous action of the eye-phic pictures cither of persons, landscapes, brows. When the object is withdrawn the sculpture, or machinery. After a little pracoptic axes open, the pupils expand, and the tice the illusion is perfect, and is aided by the eyebrows rise. ' Distance is therefore really correct geometrical perspective and chiaro seen with two eyes, and it may be proved, obscuro of the daguerreotype and the talboin opposition to the conclusions of Dr. Berke- type. To this species of relief we may give ley and many other metaphysicians, that the name of monocular, which is always indistance, whether represented by a mathe- ferior to the binocular relief in which we see matical or a physical line is visible in the original, or which is produced in the monocular as well as binocular vision. stereoscope. The relation of these three
But though relief, and distance as its rep- kinds of relief, when we look at a plane picresentative, is best seen by two eyes, yet ture, namely, ocular with two eyes, monocuvision with one eye is in the following re- lar with one, and binocular when we see spects superior to vision with two. the original solid or landscape, or its two
1. When we look at oil paintings, paint- pictures combined in the stereoscope, may ings on porcelain or any other in which the be thus observed. Look at any one of the surface is covered with a varnish, or have a binocular pictures with both eyes, and they gloss of any kind, the varnish or gloss re- have very little relief. Look at them with flects to each eye the light which falls upon it, one eye either in the stereoscope or out of from objects in various parts of the room, it, and the relief is increased. Look at them and consequently renders the picture indis. when combined in the stereoscope, and the tinct. But when we close one eye, we shut relief is perfect, and an accurate representaout the quantity of light which entered that tion of the original solid or landscape, proeye as reflected from a different part of the vided the binocular pictures have been taken room, and we consequently render the pic at the proper angle.* ture more distinct.*
3. Monocular vision is superior to bino2. A painting, picture, or photograph, cular vision, because it very frequently hapseen with one eye, is seen more perfectly pens that the one eye is less perfect than the from another cause. In these representations other, and occasionally that the one is of a upon a plane surface every part of the sur- different focal length from the other, that is, face is nearly equidistant from us, and when the two eyes see objects most distinctly at we view them with two eyes by the conver- different distances. 'In the first of these gence of the optical axes upon them, it suf cases, the imperfect image in one eye is so fers no change, the muscles of the eyeball blended or united with the perfect image in are not strained, nor the pupil alternately contracted and expanded in seeing objects at
* A large number of the binocular pictures now different distances, as is the case when we executed are not taken as if they were seen by two look at a living man, a statue, or a landscape, human eyes, but by eyes, five, ten fifteen, twenty inthe eyes being now converged in rapid suc- ches, and even many feet apart! Such pictures are cession upon the nose, eyes, and ears, or false representations of nature, and indicate the greatupon the objects in the foreground, the mid-est ignorance, and if they are not ignorant, the great
est dishonesty on the part of those who execute them. The object of the artist is to produce a startling ef
fect, and obtain a better sale for his pictures. The * The pictures in a room or gallery with side lights true method of taking binocular pictures for the should always be viewed with the eye on which no stereoscope is described and demonstrated in Sir light falls, as light diminishes the sensibility of the David Brewster's treatise, entitled, The Stereoscope : eye to the red rays, and therefore gives a false colour- its History, Theory, and Construction, with its applicaing to the picture, making all white colours of a tion to the Fine and Useful Arts, and to Education, bluish green tint.
chap. viii. London, 1856.
the other, as to give imperfect vision, and by phantasy will there prevail and blot out the only remedy for this is to shut or dis- the other." This theory was to a considercontinue the use of the imperfect eye. If the able degree anticipated by M. Rohault,* image in the bad eye is very imperfect, and with this difference, that he does not suppose its degree of luminosity very small, the pa- the nerves either to cross at the commissure tient ceases to notice it, and sees tolerably or split into two. He merely supposes that well with both open. In the second case, the two optic nerves have their correspondwhen the eyes have different focal lengths, ing or sympathetic fibres, which unite in one and are equally good, a large image is united point in the brain, and join their impressions with a small one, and the effect of this is into one, thus giving a single image from sometimes to give double pictures of objects, two formed on the retina. In this way he owing probably to an effort to put aside one not only explains single vision with two of the pictures. The only remedy for this eyes, but also the doubling of any object by defect, as we shall afterwards see, is to equal. distortion, and the impossibility of two ize the focal lengths of the two eyes by pro- things appearing in one place. In 1824 Dr. per glasses.
Wollaston reproduced the theory of NewWhen both eyes are in every respect ton, and maintained that by this theory “we equal and perfect, any imperfection in the clearly gain a step in the solution, if not a muscular apparatus by which the eyeballs full explanation of the long agitated question
direct the optical axes to the same point is of single vision with two eyes.”+ We can· most injurious, and cases have occurred in not admit the accuracy of this opinion. The which distinct vision was absolutely des- theory here referred to is not merely unnetroyed by an inability of the eyes to direct cessary, but is positively contradicted by their axes to, and fix them upon, one point. numerous facts, as the phenomenon of single This will be better understood after we have vision can be perfectly explained without explained how we see only one object with any theory whatever. two eyes. This question has been a fertile Our metaphysicians and physiologists have source of controversy among metaphysicians been as unfortunate in their explanations as and optical writers, some of whom have our optical writers. When Dr. Reid mainsought for an explanation of single vision in tains that objects appear single when their a peculiar formation of the retina from two images are formed on corresponding points optic nerves. The two optic nerves, after of the retina, and double in all other circumproceeding from the brain, cross each other stances, he gives no explanation whatever of at their commissure, a place called the Sella single vision. He merely attaches the name Turcica, where each fibre of which the nerve of corresponding points to those upon which is supposed to consist, decussates or divides the image falls when it appears single: And into two half fibres, one of which goes to the when Dr. Brown tells us that it is from asright-hand side of the retina of each eye. sociation alone that we see objects single by The terminations of these fibres in the retina means of double pictures, he merely asserts are called corresponding points, and Sir Isaac his ignorance of the cause, and his assertion Newton supposes that when the image of a is contradicted by numerous faets, and espepoint is formed on any two corresponding cially by the fact that the pictures in each points of the retina, the impression is con- eye are not similar. Dr. Alison is equally veyed along the optic nerves to their com- unfortunate in his views. After controvertmissure where they unite into one fibre, and ing the opinions of Reid and Brown, he main“concur after they have passed their junc- tains that images formed on corresponding ture, and make one image more vivid than points of the retina, naturally affect our minds one eye alone could do."* Newton adds in the same manner as a single image on the that this theory explains “ why though one retina of one eye. This explanation is simthing may appear in two places, (that is, ply a truism; for if Nature had been so perdouble) by distorting the eyes, (or pressing verse as to produce three pictures in place of one eye aside,) yet two things cannot appear one from two eyes, the result would have in one place. If the picture of one thing fall been equally natural, though inexplicable. upon one of the corresponding points, and The fallacy of all these attempts to exthe picture of another upon the other corres- plain single vision is occasioned by the false ponding point, they may both proceed to the assumption that we actually see an object commissure, but no farther. They cannot single with two eyes, whereas we only see both be carried by the same fibre into the one point of an object single with two eyes, brain; that which is strongest or most helped every other point of the object being seen
* Brewster's Memoirs of Sir Isaac Newton, vol.
* Traité de Physique, 1671.