Image unavailable: Fig. 3.Fig.3.
white membrane (the sclerotic), which encloses a nervous surface and certain refracting media (lens and 'humors') which cast a picture of the outer world thereon. It is infact a little camera obscura, the essential part of which is the sensitive plate.
Image unavailable: Fig. 4.Fig.4.
Image unavailable: Fig. 5.—Scheme of retinal fibres, after Küss. Nop. optic nerve; S, sclerotic; Ch, choroid; R, retina; P, papilla (blind spot); F, fovea.Fig.5.—Scheme of retinal fibres, after Küss. Nop. optic nerve; S, sclerotic; Ch, choroid; R, retina; P, papilla (blind spot); F, fovea.
The retinais what corresponds to this plate. The optic nerve pierces the sclerotic shell and spreads its fibres radially in every direction over its inside, forming a thin translucent film (seeFig. 3,Ret.). The fibres pass into a complicated apparatus of cells, granules, and branches (Fig. 4), and finally end in the so-called rods and cones (Fig. 4,—9), which are the specific organs for taking up the influence of the waves of light. Strange to say, these end-organs are not pointed forward towards the light as it streams through the pupil, but backwards towards the sclerotic membrane itself, so that the light-waves traverse the translucent nerve-fibres, and the cellular and granular layers of the retina, before they touch the rods and cones themselves. (SeeFig. 5.)
The Blind Spot.—The optic nerve-fibres must thus be unimpressible by light directly. The place where the nerve enters is in fact entirely blind, because nothing but fibres exist there, the other layers of the retina only beginning round about the entrance. Nothing is easier than to prove the existence of this blind spot. Close the right eye and look steadily with the left at the cross inFig. 6, holding the book vertically in front of the face, and moving it to and fro. It will be found that at about a foot off the black disk disappears; but when the page is nearer or farther, it is seen. During the experiment the gaze must be kept fixed on the cross. It is easy to show by measurement that this blind spot lies where the optic nerve enters.
Image unavailable: Fig. 6.Fig.6.
The Fovea.—Outside of the blind spot the sensibility of the retina varies. It is greatest at thefovea, a little pit lying outwardly from the entrance of the optic nerve, and round which the radiating nerve-fibres bend without passing over it. The other layers also disappear at the fovea, leaving the cones alone to represent the retina there. The sensibility of the retina grows progressively less towards its periphery, by means of which neither colors, shapes, nor number of impressions can be well discriminated.
In the normal use of our two eyes, the eyeballs are rotated so as to cause the two images of any object which catches the attention to fall on the two foveæ, as the spots of acutest vision. This happens involuntarily, as any one may observe. In fact, it is almost impossiblenotto 'turn the eyes,' the moment any peripherally lying object does catch our attention, the turning of the eyes being onlyanother name for such rotation of the eyeballs as will bring the foveæ under the object's image.
Image unavailable: Fig. 7.Fig.7.
Accommodation.—Thefocussingorsharpeningof the image is performed by a special apparatus. In every camera, the farther the object is from the eye the farther forward, and the nearer the object is to the eye the farther backward, is its image thrown. In photographers' cameras the back is made to slide, and can be drawn away from the lens when the object that casts the picture is near, and pushed forward when it is far. The picture is thus kept always sharp. But no such change of length is possible in the eyeball; and the same result is reached in another way. The lens, namely, grows more convex when a near object is looked at, and flatter when the object recedes. This change is due to the antagonism of the circular 'ligament' in which the lens is suspended, and the 'ciliary muscle.' The ligament, when the ciliary muscle is at rest, assumes such a spread-out shape as to keep the lens rather flat. But the lens is highly elastic; and it springs into the more convex form which is natural to it whenever the ciliary muscle, by contracting, causes the ligament to relax its pressure. The contraction of the muscle, by thus rendering the lens more refractive, adapts the eye for near objects ('accommodates' it for them, as we say); and its relaxation, by rendering the lens less refractive, adapts the eye for distant vision. Accommodation for the near is thusthe moreactivechange, since it involves contraction of the ciliary muscle. When we look far off, we simply let our eyes go passive. We feel this difference in the effort when we compare the two sensations of change.
Convergence accompanies accommodation.The two eyes act as one organ; that is, when an object catches the attention, both eyeballs turn so that its images may fall on the foveæ. When the object is near, this naturally requires them to turn inwards, or converge; and as accommodation then also occurs, the two movements of convergence and accommodation form a naturally associated couple, of which it is difficult to execute either singly. Contraction of the pupil also accompanies the accommodative act. When we come to stereoscopic vision, it will appear that by much practice one can learn to converge with relaxed accommodation, and to accommodate with parallel axes of vision. These are accomplishments which the student of psychological optics will find most useful.
Single Vision by the two Retinæ.—We hear single with two ears, and smell single with two nostrils, and we also see single with two eyes. The difference is that we alsocansee double under certain conditions, whereas under no conditions can we hear or smell double. The main conditions of single vision can be simply expressed.
In the first place, impressions on the two foveæ always appear in the same place. By no artifice can they be made to appear alongside of each other. The result is that one object, casting its images on the foveæ of the two converging eyeballs will necessarily always appear as what it is, namely, one object. Furthermore, if the eyeballs, instead of converging, are kept parallel, and two similar objects, one in front of each, cast their respective images on the foveæ, the two will also appear as one, or (in common parlance) 'their images will fuse.' To verify this, let the reader stare fixedly before him as if through the paper at infinite distance, with the black spots in Fig. 8 in front of his respective eyes. Hewill then see the two black spots swim together, as it were, and combine into one, which appears situated between their original two positions and as if opposite the root of his nose. This combined spot is the result of the spots opposite both eyes being seen in the same place. But in addition to the combined spot, each eye sees also the spot opposite theothereye. To the right eye this appears to the left of the combined spot, to the left eye it appears to the right of it; so that what is seen isthreespots, of which the middle one is seen by both eyes, and is flanked by two others, each seen by one. That such are the facts can be tested by interposing some small opaque object so as to cut off the vision of either of the spots in the figure from theothereye. A vertical partition in the median plane, going from the paper to the nose, will effectually confine each eye's vision to the spot in front of it, and then the single combined spot will be all that appears.[11]
Image unavailable: Fig. 8.Fig.8.
If, instead of two identical spots, we use two different figures, or two differently colored spots, as objects for the two foveæ to look at, they still are seen in thesame place; but since they cannot appear as a single object, they appear therealternatelydisplacing each other from the view. This is the phenomenon calledretinal rivalry.
As regards the parts of the retinæ round about the foveæ, a similar correspondence obtains. Any impression on theupper half of either retina makes us see an object as below, on the lower half as above, the horizon; and on the right half of either retina, an impression makes us see an object to the left, on the left half one to the right, of the median line. Thus each quadrant of one retina corresponds as a whole to the geometricallysimilarquadrant of the other; and within two similar quadrants,alandarfor example, there should, if the correspondence were carried out in detail, be geometrically similar points which, if impressed at the same time by light emitted from the same object, should cause that object to appear in the same direction to either eye. Experiment verifies this surmise. If we look at the starry vault with parallel eyes, the stars all seem single; and the laws of perspective show that under the circumstances the parallel light-rays coming from each star must impinge on points within either retina whicharegeometrically similar to each other. Similarly, a pair of spectacles held an inch or so from the eyes seem like one large median glass. Or we may make an experiment like that with the spots. If we take two exactly similar pictures, no larger than those on an ordinary stereoscopic slide, and if we look at one with each eye (a median partition confining the view) we shall see but one flat picture, all of whose parts appear single. 'Identical retinal points' being impressed, both eyes see their object in the same direction, and the two objects consequently coalesce into one.
Image unavailable: Fig. 9.Fig.9.
Here again retinal rivalry occurs if the pictures differ. And it must be noted that when the experiment is performedfor the first time the combined picture is always far from sharp. This is due to the difficulty mentioned onp. 33, of accommodating for anything as near as the surface of the paper, whilst at the same time the convergence is relaxed so that each eye sees the picture in front of itself.
Image unavailable: Fig. 10.Fig.10.
Double Images.—Now it is an immediate consequence of the law of identical location of images falling on geometrically similar points thatimages which fall upon geometricallyDISPARATEpoints of the two retinæ should be seen inDISPARATEdirections, and that their objects should consequently appear inTWOplaces, orLOOK DOUBLE. Take the parallel rays from a star falling upon two eyes which converge upon a near object,O, instead of being parallel as in the previously instanced case. The two foveæ will receive the images ofO, which therefore will look single. If thenSLandSRin Fig. 10 be the parallel rays, each of them will fall upon the nasal half of the retinawhich it strikes. But the two nasal halves are disparate, geometricallysymmetrical, not geometricallysimilar. The star's image on the left eye will therefore appear as if lying to the left ofO; its image on the right eye will appear to the right of this point. The star will, in short, be seen double—'homonymously' double.
Conversely, if the star be looked at directly with parallel axes, any near object likeOwill be seen double, because its images will affect the outer or cheek halves of the two retinæ, instead of one outer and one nasal half. The position of the images will here be reversed from that of the previous case. The right eye's image will now appear to the left, the left eye's to the right; the double images will be 'heteronymous.'
The same reasoning and the same result ought to apply where the object's place with respect to the direction of the two optic axes is such as to make its images fall not on non-similar retinal halves, but on non-similar parts of similar halves. Here, of course, the positions seen will be less widely disparate than in the other case, and the double images will appear to lie less widely apart.
Careful experiments made by many observers according to the so-called haploscopic method confirm this law, and show thatcorresponding points, of single visual direction, exist upon the two retinæ. For the detail of these one must consult the special treatises.
Vision of Solidity.—This description of binocular vision follows what is called the theory of identical points. On the whole it formulates the facts correctly. The only odd thing is that we should be so little troubled by the innumerable double images which objects nearer and farther than the point looked at must be constantly producing. The answer to this is thatwe have trained ourselves to habits of inattentionin regard to double images. So far as things interest us we turn our foveæ upon them, and they are necessarily seen single; so that if an object impresses disparate points, that may be taken as proof that it is sounimportant for us that we needn't notice whether it appears in one place or in two. By long practice one may acquire great expertness in detecting double images, though, as some one says, it is an art which is not to be learned completely either in one year or in two.
Image unavailable: Fig. 11.Fig.11.
Where the disparity of the images is but slight it is almost impossible to see them as if double. They give rather the perception of a solid object being there. To fix our ideas, takeFig. 11.Suppose we look at the dots in the middle of the linesaandbjust as we looked at the spots inFig. 8.We shall get the same result—i.e., they will coalesce in the median line. But the entire lines will not coalesce, for, owing to their inclination, their tops fall on the temporal, and their bottoms on the nasal, retinal halves. What we see will be two lines crossed in the middle, thus (Fig. 12):
Image unavailable: Fig. 12.Fig.12.
The moment we attend to the tops of these lines, however, our foveæ tend to abandon the dots and to move upwards, and in doing so, to converge somewhat, following the lines, which then appear coalescing at the top as inFig. 13.
Image unavailable: Fig. 13.Fig. 13.Fig. 14.
If we think of the bottom, the eyes descend and diverge, and what we see isFig. 14.
Running our eyes up and down the lines makes them converge and diverge just as they would were they runningup and down some single line whose top was nearer to us than its bottom. Now, if the inclination of the lines be moderate, we may not see them double at all, but single throughout their length, when we look at the dots. Under these conditions their top does look nearer than their bottom—in other words, we see them stereoscopically; and we see them so even when our eyes are rigorously motionless. In other words, the slight disparity in the bottom-ends whichwoulddraw the foveæ divergently apart makes us see those ends farther, the slight disparity in the top ends whichwoulddraw them convergently together makes us see these ends nearer, than the point at which we look. The disparities, in short, affect our perception as the actual movements would.[12]
The Perception of Distance.—When we look about us at things, our eyes are incessantly moving, converging, diverging, accommodating, relaxing, and sweeping over the field. The field appears extended in three dimensions, with some of its parts more distant and some more near.
"With one eye our perception of distance is very imperfect, as illustrated by the common trick of holding a ring suspended by a string in front of a person's face, and telling him to shut one eye and pass a rod from one side through the ring. If a penholder be held erect before one eye, while the other is closed, and an attempt be made to touch it with a finger moved across towards it, an error will nearly always be made. In such cases we get the only clue from the amount of effort needed to 'accommodate' the eye to see the object distinctly. When we use both eyes our perception of distance is much better; when we look at an object with two eyes the visual axes are converged on it, and the nearer the object the greater the convergence. We have a pretty accurate knowledge of the degree of muscular effort required to converge the eyes on all tolerably near points. When objects arefarther off, their apparent size, and the modifications their retinal images experience by aërial perspective, come in to help. The relative distance of objects is easiest determined by moving the eyes; all stationary objects then appear displaced in the opposite direction (as for example when we look out of the window of a railway car) and those nearest most rapidly; from the different apparent rates of movement we can tell which are farther and which nearer."[13]
"With one eye our perception of distance is very imperfect, as illustrated by the common trick of holding a ring suspended by a string in front of a person's face, and telling him to shut one eye and pass a rod from one side through the ring. If a penholder be held erect before one eye, while the other is closed, and an attempt be made to touch it with a finger moved across towards it, an error will nearly always be made. In such cases we get the only clue from the amount of effort needed to 'accommodate' the eye to see the object distinctly. When we use both eyes our perception of distance is much better; when we look at an object with two eyes the visual axes are converged on it, and the nearer the object the greater the convergence. We have a pretty accurate knowledge of the degree of muscular effort required to converge the eyes on all tolerably near points. When objects arefarther off, their apparent size, and the modifications their retinal images experience by aërial perspective, come in to help. The relative distance of objects is easiest determined by moving the eyes; all stationary objects then appear displaced in the opposite direction (as for example when we look out of the window of a railway car) and those nearest most rapidly; from the different apparent rates of movement we can tell which are farther and which nearer."[13]
Subjectively considered, distance is an altogether peculiar content of consciousness. Convergence, accommodation, binocular disparity, size, degree of brightness, parallax, etc., all give us special feelings which aresignsof the distance feeling, but not it. They simply suggest it to us. The best way to get it strongly is to go upon some hill-top and invert one's head. The horizon then looks very distant, and draws near as the head erects itself again.
The Perception of Size.—"The dimensions of the retinal image determine primarily the sensations on which conclusions as to size are based; and the larger the visual angle the larger the retinal image: since the visual angle depends on the distance of an object, the correct perception of size depends largely upon a correct perception of distance; having formed a judgment, conscious or unconscious, as to that, we conclude as to size from the extent of the retinal region affected. Most people have been surprised now and then to find that what appeared a large bird in the clouds was only a small insect close to the eye; the large apparent size being due to the previous incorrect judgment as to the distance of the object. The presence of an object of tolerably well-known height, as a man, also assists in forming conceptions (by comparison) as to size; artists for this purpose frequently introduce human figures to assist in giving an idea of the size of other objects represented."[14]
Sensations of Color.—The system of colors is a very complex thing. If one take any color, say green, one can passaway from it in more than one direction, through a series of greens more and more yellowish, let us say, towards yellow, or through another series more and more bluish towards blue. The result would be that if we seek to plot out on paper the various distinguishable tints, the arrangement cannot be that of a line, but has to cover a surface. With the tints arranged on a surface we can pass from any one of them to any other by various lines of gradually changing intermediaries. Such an arrangement is represented inFig. 15.It is a merely classificatory diagram based on degrees of difference simply felt, and has no physical significance. Black is a color, but does not figure on the plane of the diagram. We cannot place it anywhere alongside of the other colors because we need both to represent the straight gradation from untinted white to black, and that from each pure color towards black as well as towards white. The best way is to put black into the third dimension, beneath the paper,e.g., as is shown perspectively inFig. 16, then all the transitions can be schematically shown. One can pass straight from black to white, or one can pass round by way of olive, green, and pale green; or one can change from dark blue to yellow through green, or by way of sky-blue, white and straw color; etc., etc. In any case the changes are continuous; and the color system thus forms what Wundt calls a tri-dimensional continuum.
Image unavailable: Fig. 15.Fig.15.
Color-mixture.—Physiologically considered, the colors have this peculiarity, that many pairs of them, when they impress the retina together, produce the sensation of white. The colors which do this are calledcomplementaries. Such are spectral red and green-blue, spectral yellow and indigo-blue. Green and purple, again, are complementaries. Allthe spectral colors added together also make white light, such as we daily experience in the sunshine. Furthermore, both homogeneous ether-waves and heterogeneous ones may make us feel the same color, when they fall on our retina. Thus yellow, which is a simple spectral color, is also felt when green light is added to red; blue is felt when violet and green lights are mixed. Purple, which is not a spectral color at all, results when the waves either of red and of violet or those of blue and of orange are superposed.[15]
Image unavailable: Fig. 16 (after Ziehen).Fig.16 (after Ziehen).
From all this it follows that there is no particular congruence between our system of color-sensations and the physical stimuli which excite them. Each color-feeling is a 'specific energy' (p. 11) which many different physical causes may arouse. Helmholtz, Hering, and others have sought to simplify the tangle of the facts, by physiological hypotheses which, differing much in detail, agree in principle, since they all postulate a limited number of elementary retinal processes to which, when excited singly,certain 'fundamental' colors severally correspond. When excited in combination, as they may be by the most various physical stimuli, other colors, called 'secondary,' are felt. The secondary color-sensations are often spoken of as if they were compounded of the primary sensations. This is a great mistake. Thesensations as suchare not compounded—yellow, for example, a secondary on Helmholtz's theory, is as unique a quality of feeling as the primaries red and green, which are said to 'compose' it. What are compounded are merely the elementary retinal processes. These, according to their combination, produce diverse results on the brain, and thence the secondary colors result immediately in consciousness. The 'color-theories' are thus physiological, not psychological, hypotheses, and for more information concerning them the reader must consult the physiological books.
The Duration of Luminous Sensations.—"This is greater than that of the stimulus, a fact taken advantage of in making fireworks: an ascending rocket produces the sensation of a trail of light extending far behind the position of the bright part of the rocket itself at the moment, because the sensation aroused by it in a lower part of its course still persists. So, shooting stars appear to have luminous tails behind them. By rotating rapidly before the eye a disk with alternate white and black sectors we get for each point of the retina alternate stimulation (due to the passage of white sector) and rest (when a black sector is passing). If the rotation be rapid enough the sensation aroused is that of a uniform gray, such as would be produced if the white and black were mixed and spread evenly over the disk. In each revolution the eye gets asmuch light as if that were the case, and is unable to distinguish that this light is made up of separate portions reaching it at intervals: the stimulation due to each lasts until the next begins, and so all are fused together. If one turns out suddenly the gas in a room containing no other light, the image of the flame persists a short time after the flame itself is extinguished."[16]If we open our eyes instantaneously upon a scene, and then shroud them in complete darkness, it will be as if we saw the scene in ghostly light through the dark screen. We can read off details in it which were unnoticed whilst the eyes were open. This is the primary positive after-image, so-called. According to Helmholtz, one third of a second is the most favorable length of exposure to the light for producing it.
Negative after-imagesare due to more complex conditions, in which fatigue of the retina is usually supposed to play the chief part.
"The nervous visual apparatus is easily fatigued. Usually we do not observe this because its restoration is also rapid, and in ordinary life our eyes, when open, are never at rest; we move them to and fro, so that parts of the retina receive light alternately from brighter and darker objects, and are alternately excited and rested. How constant and habitual the movement of the eyes is can be readily observed by trying to 'fix' for a short time a small spot without deviating the glance; to do so for even a few seconds is impossible without practice. If any small object is steadily 'fixed' for twenty or thirty seconds, it will be found that the whole field of vision becomes grayish and obscure, because the parts of the retina receiving most light get fatigued, and arouse no more sensation than those less fatigued and stimulated by light from less illuminated objects. Or look steadily at a black object, say a blot on a white page, for twenty seconds, and then turn the eye on a white wall; the latter will seem dark gray, with a white patch on it; an effect due to the greater excitability of the retinal parts previously rested by the black, when compared with the sensation aroused elsewhere by light from the white wall acting on the previously stimulated parts of the visual surface. All persons will recall many instances of such phenomena, which are especially noticeable soon after rising in the morning.Similar things may be noticed with colors; after looking at a red patch the eye turned on a white wall sees a blue-green patch; the elements causing red sensations having been fatigued, the white mixed light from the wall now excites on that region of the retina only the other primary color sensations. The blending of colors so as to secure their greatest effect depends on this fact; red and green go well together because each rests the parts of the visual apparatus most excited by the other, and so each appears bright and vivid as the eye wanders to and fro; while red and orange together, each exciting and exhausting mainly the same visual elements, render dull, or in popular phrase 'kill,' one another."If we fix steadily for thirty seconds a point between two white squares about 4 mm. (⅙ inch) apart on a large black sheet, and then close and cover our eyes, we get a negative after-image in which are seen two dark squares on a brighter surface; this surface is brighter close around the negative after-image of each square, and brightest of all between them. This luminous boundary is called thecorona, and is explained usually as an effect of simultaneous contrast; the dark after-image of the square it is said makes us mentally err in judgment, and think the clear surface close to it brighter than elsewhere; and it is brightest between the two dark squares, just as a middle-sized man between two tall ones looks shorter than if alongside one only. If, however, the after-image be watched, it will often be noticed not only that the light band between the squares is intensely white, much more so than the normal idio-retinal light [see below], but, as the image fades away, often the two dark after-images of the squares disappear entirely with all of the corona, except that part between them which is still seen as a bright band on a uniform grayish field. Here there is nocontrastto produce the error of judgment; and from this and other experiments Hering concludes that light acting on one part of the retina produces inverse changes in all the rest, and that this plays an important part in producing the phenomena of contrasts. Similar phenomena may be observed with colored objects; in their negative after-images each tint is represented by its complementary, as black is by white in colorless vision."[17]
"The nervous visual apparatus is easily fatigued. Usually we do not observe this because its restoration is also rapid, and in ordinary life our eyes, when open, are never at rest; we move them to and fro, so that parts of the retina receive light alternately from brighter and darker objects, and are alternately excited and rested. How constant and habitual the movement of the eyes is can be readily observed by trying to 'fix' for a short time a small spot without deviating the glance; to do so for even a few seconds is impossible without practice. If any small object is steadily 'fixed' for twenty or thirty seconds, it will be found that the whole field of vision becomes grayish and obscure, because the parts of the retina receiving most light get fatigued, and arouse no more sensation than those less fatigued and stimulated by light from less illuminated objects. Or look steadily at a black object, say a blot on a white page, for twenty seconds, and then turn the eye on a white wall; the latter will seem dark gray, with a white patch on it; an effect due to the greater excitability of the retinal parts previously rested by the black, when compared with the sensation aroused elsewhere by light from the white wall acting on the previously stimulated parts of the visual surface. All persons will recall many instances of such phenomena, which are especially noticeable soon after rising in the morning.Similar things may be noticed with colors; after looking at a red patch the eye turned on a white wall sees a blue-green patch; the elements causing red sensations having been fatigued, the white mixed light from the wall now excites on that region of the retina only the other primary color sensations. The blending of colors so as to secure their greatest effect depends on this fact; red and green go well together because each rests the parts of the visual apparatus most excited by the other, and so each appears bright and vivid as the eye wanders to and fro; while red and orange together, each exciting and exhausting mainly the same visual elements, render dull, or in popular phrase 'kill,' one another.
"If we fix steadily for thirty seconds a point between two white squares about 4 mm. (⅙ inch) apart on a large black sheet, and then close and cover our eyes, we get a negative after-image in which are seen two dark squares on a brighter surface; this surface is brighter close around the negative after-image of each square, and brightest of all between them. This luminous boundary is called thecorona, and is explained usually as an effect of simultaneous contrast; the dark after-image of the square it is said makes us mentally err in judgment, and think the clear surface close to it brighter than elsewhere; and it is brightest between the two dark squares, just as a middle-sized man between two tall ones looks shorter than if alongside one only. If, however, the after-image be watched, it will often be noticed not only that the light band between the squares is intensely white, much more so than the normal idio-retinal light [see below], but, as the image fades away, often the two dark after-images of the squares disappear entirely with all of the corona, except that part between them which is still seen as a bright band on a uniform grayish field. Here there is nocontrastto produce the error of judgment; and from this and other experiments Hering concludes that light acting on one part of the retina produces inverse changes in all the rest, and that this plays an important part in producing the phenomena of contrasts. Similar phenomena may be observed with colored objects; in their negative after-images each tint is represented by its complementary, as black is by white in colorless vision."[17]
This is one of the facts referred to onp. 27which have made Hering reject the psychological explanation of simultaneous contrast.
The Intensity of Luminous Objects.—Black is an optical sensation. We have no black except in the field of view;we do not, for instance, see black out of our stomach or out of the palm of our hand.Pureblack is, however, only an 'abstract idea,' for the retina itself (even in complete objective darkness) seems to be always the seat of internal changes which give some luminous sensation. This is what is meant by the 'idio-retinal light,' spoken of a few lines back. It plays its part in the determination of all after-images with closed eyes. Any objective luminous stimulus, to be perceived, must be strong enough to give a sensible increment of sensation over and above the idio-retinal light. As the objective stimulus increases the perception is of an intenser luminosity; but the perception changes, as we saw onp. 18, more slowly than the stimulus. The latest numerical determinations, by König and Brodhun, were applied to six different colors and ran from an intensity arbitrarily called 1 to one which was 100,000 times as great. From intensity 2000 to 20,000 Weber's law held good; below and above this range discriminative sensibility declined. The relative increment discriminated here was the same for all colors of light, and lay (according to the tables) between 1 and 2 per cent of the stimulus. Previous observers have got different results.
A certain amount of luminous intensity must exist in an object for its color to be discriminated at all. "In the dark all cats are gray." But the colors rapidly become distincter as the light increases, first the blues and last the reds and yellows, up to a certain point of intensity, when they grow indistinct again through the fact that each takes a turn towards white. At the highest bearable intensity of the light all colors are lost in the blinding white dazzle. This again is usually spoken of as a 'mixing' of the sensation white with the original color-sensation. It is no mixing of two sensations, but the replacement of one sensation by another, in consequence of a changed neural process.
Image unavailable: Fig. 17.—Semidiagrammatic section through the right ear (Czermak). M, concha; G, external auditory meatus; T, tympanic membrane; P, tympanic cavity; o, oval foramen; r, round foramen; R, pharyngeal opening of Eustachian tube; V, vestibule; B, a semicircular canal; S, the cochlea; Vt, scala vestibuli; Pt, scala tympani; A, auditory nerve.Fig.17.—Semidiagrammatic section through the right ear (Czermak). M, concha; G, external auditory meatus; T, tympanic membrane; P, tympanic cavity; o, oval foramen; r, round foramen; R, pharyngeal opening of Eustachian tube; V, vestibule; B, a semicircular canal; S, the cochlea; Vt, scala vestibuli; Pt, scala tympani; A, auditory nerve.
Fig.17.—Semidiagrammatic section through the right ear (Czermak). M, concha; G, external auditory meatus; T, tympanic membrane; P, tympanic cavity; o, oval foramen; r, round foramen; R, pharyngeal opening of Eustachian tube; V, vestibule; B, a semicircular canal; S, the cochlea; Vt, scala vestibuli; Pt, scala tympani; A, auditory nerve.
Fig.17.—Semidiagrammatic section through the right ear (Czermak). M, concha; G, external auditory meatus; T, tympanic membrane; P, tympanic cavity; o, oval foramen; r, round foramen; R, pharyngeal opening of Eustachian tube; V, vestibule; B, a semicircular canal; S, the cochlea; Vt, scala vestibuli; Pt, scala tympani; A, auditory nerve.
Fig.17.—Semidiagrammatic section through the right ear (Czermak). M, concha; G, external auditory meatus; T, tympanic membrane; P, tympanic cavity; o, oval foramen; r, round foramen; R, pharyngeal opening of Eustachian tube; V, vestibule; B, a semicircular canal; S, the cochlea; Vt, scala vestibuli; Pt, scala tympani; A, auditory nerve.
The Ear.—"The auditory organ in man consists of three portions, known respectively as theexternal ear, themiddle earortympanum, and theinternal earorlabyrinth; the latter contains the end-organs of the auditory nerve. The external ear consists of the expansion seen on the exterior of the head, called theconcha,M,Fig. 17,and a passage leading in from it, theexternal auditory meatus,G. This passage is closed at its inner end by thetympanicordrum membrane,T. It is lined by skin, through which numerous small glands, secreting thewaxof the ear, open.
Image unavailable: Fig. 18.—Mcp, Mc, Ml, and Mm stand for different parts of the malleus; Jc, Jb, Jl, Jpl, for different parts of the incus. S is the stapes.Fig.18.—Mcp, Mc, Ml, and Mm stand for different parts of the malleus; Jc, Jb, Jl, Jpl, for different parts of the incus. S is the stapes.
Fig.18.—Mcp, Mc, Ml, and Mm stand for different parts of the malleus; Jc, Jb, Jl, Jpl, for different parts of the incus. S is the stapes.
Fig.18.—Mcp, Mc, Ml, and Mm stand for different parts of the malleus; Jc, Jb, Jl, Jpl, for different parts of the incus. S is the stapes.
Fig.18.—Mcp, Mc, Ml, and Mm stand for different parts of the malleus; Jc, Jb, Jl, Jpl, for different parts of the incus. S is the stapes.
"The Tympanum(P,Fig. 17) is an irregular cavity in the temporal bone, closed externally by the drum membrane. From its inner side theEustachian tube(R) proceeds and opens into the pharynx. The inner wall of the tympanum is bony except for two small apertures, theovalandround foramens,oandr, which lead into the labyrinth. During life the round aperture is closed by the lining mucous membrane, and the oval by the stirrup-bones. Thetympanic membraneT, stretched across the outer side of the tympanum, forms a shallow funnel with its concavity outwards. It is pressed by the external air on its exterior, and by air entering the tympanic cavity through the Eustachian tube on its inner side. If the tympanum were closed these pressures would not be always equal when barometric pressure varied, and the membrane would be bulged in or out according as the external or internal pressure on it were the greater. On the other hand, were the Eustachian tube always open the sounds of our own voices would be loud and disconcerting, so it is usually closed; but every time we swallow it is opened, and thus the air-pressure in the cavity is kept equal to that in the external auditory meatus. On making a balloon ascent or going rapidly down a deep mine, the sudden and great change of aërial pressure outside frequently causespainful tension of the drum-membrane, which may be greatly alleviated by frequent swallowing.
The Auditory Ossicles.—Three small bones lie in the tympanum forming a chain from the drum-membrane to the oval foramen. The external bone is themalleusorhammer; the middle one, theincusoranvil; and the internal one, thestapesorstirrup. They are represented inFig. 18.[19]
Accommodationis provided for in the ear as well as in the eye. One muscle an inch long, thetensor tympani, arises in the petrous portion of the temporal bone (running in a canal parallel to the Eustachian tube) and is inserted into the malleus below its head. When it contracts, it makes the membrane of the tympanum more tense. Another smaller muscle, thestapedius, goes to the head of the stirrup-bone. These muscles are by many persons felt distinctly contracting when certain notes are heard, and some can make them contract at will. In spite of this, uncertainty still reigns as to their exact use in hearing, though it is highly probable that they give to the membranes which they influence the degree of tension best suited to take up whatever rates of vibration may fall upon them at the time. In listening, the head and ears in lower animals, and the head alone in man, are turned so as best to receive the sound. This also is a part of the reaction called 'adaptation' of the organ (see the chapter on Attention).
The Internal Ear.—"The labyrinth consists primarily of chambers and tubes hollowed out in the temporal bone and inclosed by it on all sides, except for the oval and round foramens on its exterior, and certain apertures for blood-vessels and the auditory nerve; during life all these are closed water-tight in one way or another. Lying in thebony labyrinththus constituted are membranous parts, of the same general form but smaller, so that between the twoa space is left; this is filled with a watery fluid, called theperilymph; and themembranous internal earis filled by a similar liquid, theendolymph.
Image unavailable: Fig. 19.—Casts of the bony labyrinth. A, left labyrinth seen from the outer side; B, right labyrinth from the inner side; C, left labyrinth from above; Co, cochlea; V, vestibule; Fc, round foramen; Fv, oval foramen; h, horizontal semicircular canal; ha, its ampulla; vaa, ampulla of anterior vertical semicircular canal; vpa, ampulla of posterior vertical semicircular canal; vc, conjoined portion of the two vertical canals.Fig.19.—Casts of the bony labyrinth. A, left labyrinth seen from the outer side; B, right labyrinth from the inner side; C, left labyrinth from above; Co, cochlea; V, vestibule; Fc, round foramen; Fv, oval foramen; h, horizontal semicircular canal; ha, its ampulla; vaa, ampulla of anterior vertical semicircular canal; vpa, ampulla of posterior vertical semicircular canal; vc, conjoined portion of the two vertical canals.
Fig.19.—Casts of the bony labyrinth. A, left labyrinth seen from the outer side; B, right labyrinth from the inner side; C, left labyrinth from above; Co, cochlea; V, vestibule; Fc, round foramen; Fv, oval foramen; h, horizontal semicircular canal; ha, its ampulla; vaa, ampulla of anterior vertical semicircular canal; vpa, ampulla of posterior vertical semicircular canal; vc, conjoined portion of the two vertical canals.
Fig.19.—Casts of the bony labyrinth. A, left labyrinth seen from the outer side; B, right labyrinth from the inner side; C, left labyrinth from above; Co, cochlea; V, vestibule; Fc, round foramen; Fv, oval foramen; h, horizontal semicircular canal; ha, its ampulla; vaa, ampulla of anterior vertical semicircular canal; vpa, ampulla of posterior vertical semicircular canal; vc, conjoined portion of the two vertical canals.
Fig.19.—Casts of the bony labyrinth. A, left labyrinth seen from the outer side; B, right labyrinth from the inner side; C, left labyrinth from above; Co, cochlea; V, vestibule; Fc, round foramen; Fv, oval foramen; h, horizontal semicircular canal; ha, its ampulla; vaa, ampulla of anterior vertical semicircular canal; vpa, ampulla of posterior vertical semicircular canal; vc, conjoined portion of the two vertical canals.
The Bony Labyrinth.—"The bony labyrinth is described in three portions, thevestibule, thesemicircular canals, and thecochlea; casts of its interior are represented from different aspects inFig. 19.The vestibule is the central part and has on its exterior the oval foramen (Fv) into which the base of the stirrup-bone fits. Behind the vestibule are three bony semicircular canals, communicating with the back of the vestibule at each end, and dilated near one end to form anampulla. The bony cochlea is a tube coiled on itself somewhat like a snail's shell, and lying in front of the vestibule.
The Membranous Labyrinth.—"The membranous vestibule, lying in the bony, consists of two sacs communicating by a narrow aperture. The posterior is called theutriculus, and into it the membranous semicircular canals open. The anterior, called thesacculus, communicates by a tube with the membranous cochlea. The membranous semicircular canals much resemble the bony, and each has
Image unavailable: Fig. 20.—A section through the cochlea in the line of its axis.Fig.20.—A section through the cochlea in the line of its axis.
Fig.20.—A section through the cochlea in the line of its axis.
Fig.20.—A section through the cochlea in the line of its axis.
Fig.20.—A section through the cochlea in the line of its axis.
Image unavailable: Fig. 21.—Section of one coil of the cochlea, magnified. SV, scala vestibuli; R, membrane of Reissner; CC, membranous cochlea (scala media); lls, limbus laminæ spiralis; t, tectorial membrane; ST, scala tympani; lso, spiral lamina; Co, rods of Corti; b, basilar membrane.Fig.21.—Section of one coil of the cochlea, magnified. SV, scala vestibuli; R, membrane of Reissner; CC, membranous cochlea (scala media); lls, limbus laminæ spiralis; t, tectorial membrane; ST, scala tympani; lso, spiral lamina; Co, rods of Corti; b, basilar membrane.
Fig.21.—Section of one coil of the cochlea, magnified. SV, scala vestibuli; R, membrane of Reissner; CC, membranous cochlea (scala media); lls, limbus laminæ spiralis; t, tectorial membrane; ST, scala tympani; lso, spiral lamina; Co, rods of Corti; b, basilar membrane.
Fig.21.—Section of one coil of the cochlea, magnified. SV, scala vestibuli; R, membrane of Reissner; CC, membranous cochlea (scala media); lls, limbus laminæ spiralis; t, tectorial membrane; ST, scala tympani; lso, spiral lamina; Co, rods of Corti; b, basilar membrane.
Fig.21.—Section of one coil of the cochlea, magnified. SV, scala vestibuli; R, membrane of Reissner; CC, membranous cochlea (scala media); lls, limbus laminæ spiralis; t, tectorial membrane; ST, scala tympani; lso, spiral lamina; Co, rods of Corti; b, basilar membrane.
an ampulla; in the ampulla one side of the membranous tube is closely adherent to its bony protector; at this point nerves enter the former. The relations of the membranous to the bony cochlea are more complicated. A section through this part of the auditory apparatus (Fig. 20) shows that its osseous portion consists of a tube wound two and a half times round a central bony axis, themodiolus. From the axis a shelf, thelamina spiralis, projects and partially subdivides the tube, extending farthest across in its lower coils. Attached to the outer edge of this bony plate is the membranous cochlea (scala media), a tube triangular in cross-section and attached by its base to the outer side of the bony cochlear spiral. The spiral lamina and the membranous cochlea thus subdivide the cavity of the bony tube (Fig. 21) into an upper portion, thescala vestibuli,SV, and a lower, thescala tympani,ST. Between these lie the lamina spiralis (lso) and the membranouscochlea (CC), the latter being bounded above by the membrane of Reissner (R) and below by the basilar membrane (b)."[20]
The membranous cochlea does not extend to the tip of the bony cochlea; above its apex the scala vestibuli and scala tympani communicate. Both are filled with perilymph, so that when the stapes is pushed into the oval foramen,o, inFig. 17, by the impact of an air-wave on the tympanic membrane, a wave of perilymph runs up the scala vestibuli to the top, where it turns into the scala tympani, down whose whorls it runs and pushes out the round foramenr, ruffling probably the membrane of Reissner and the basilar membrane on its way up and down.