Billeder på siden



45° below the horizontal line. The eye, in passing from one part of the field to another, might be supposed to take the straight route. Wundt is of opinion that the straight route is preferred only in the horizontal and the vertical sweep. In other directions, the sweep is in a curve, which is greatest when the two points in the field of vision make an angle of 45° with the horizon.

3. Such being the mechanism of the eye, I must now touch briefly upon its mode of acting as the organ of sight. The optical part of the process is well enough understood. When the eye is directed to any object, an image of that object is depicted on the back of the eye, by means of the rays of light entering the pupil, and duly refracted by the different humours. The precise mode of stimulating the nervous filaments of the retina is not known; but the pigment cells of the choroid play an important part, being themselves highly absorbent of light; where they are not found, as at the place of entrance of the optic nerve, there is no power of vision (the blind spot). In order to perfect vision the following farther conditions are necessary :—

(1.) A sufficiency of light or illumination in the object viewed. This is an obvious necessity. We judge of the quantity of light present by the power we have of seeing objects distinctly. Some animals can see with much less light than others, and to such the noonday sun must be painful.

(2.) The formation of the image exactly on the retina, and not before or behind. The focus of the image must coincide with the retina. If this is not the case the image is indistinct; the rays of light either do not converge, or have begun to disperse, at the back of the eye. The perfect convergence of the image by the lenses constituting the ball of the eye, depends on the distance of the object, and also in some degree on the self-adjustment of the eye. As this power of adaptation of the eye itself for vision, at different distances, has its limits, there is in every individual a distance at which he sees most distinctly, and at which the focus of the image, formed by the refracting media of the eye, corre

sponds most accurately with the situation of the retina. This distance may be stated at from five to ten inches, in the majority of individuals. Objects which are too near the eye throw very indistinct images upon the retina; a slender body, such as a pin, held close to the eye, cannot be seen at all, or produces only an undefined impression on the retina. Few persons, on the other hand, are able to read print at a much greater distance than twenty inches.'

(3.) The third condition of perfect vision is the minute size of the subdivisions of the retina capable of independent sensation. We are sensitive to very minute lines and points; and there is a limit of minuteness, where a number of distinct lines would seem as one. This is the limit of the optical subdivision of the retina, analogous to the intervals of double sensation in touch.

It appears that minuteness of discrimination is aided by the following circumstances. 1. An intense light will enable a smaller object to be seen. 2. A white picture can be seen smaller than a blue. 3. A line can be seen better than a point of the same diameter. The smallest angle for a round body is 20"; a thread-like object is discernible under an angle of 3"; a glancing wire can impress the eye at an angle of ". According to Weber and Volkmann, two bright lines must be separated at least from adoo to redoʊ of an inch on the yellow spot to give a double sensation; which is an estimate quite compatible with the observed minuteness of the fibres and vesicles of the retina, supposing each of these capable of conveying an independent impression to the brain.

The power of discrimination diminishes rapidly as the impression recedes from the yellow spot. At a point 60° from the centre of the spot, an object must be 150 times larger, in order to be distinguished. Thus, although the eye can take in a wide field at once, the power of minute observation is confined to a very small part in the centre of the retina.*

Another condition of perfect vision has been suggested by the following experiments of Wundt:-If a small piece of red paper is held before the eye, and then moved to one side without the eye following it, so that the impres



The great superiority of the eye, as a medium for perceiving the outer world, lies in this power of independent sensibility to minute points. I have already adverted to the distinction between the lower and the higher senses in this particular. The nerve of vision must needs consist of a number of independent fibres maintaining their distinctness all the way to the brain, and capable of causing distinct waves of diffusion throughout the entire cerebral mass; every one of these many thousand impressions varying the mental

sion is made first in the yellow spot, and then on the lateral parts of the retina, the colour is variously seen. To the yellow spot, the paper is red; as it moves sideways, it becomes darker; gradually, it assumes a blueish tint, and, at last, it appears perfectly black. Similar variations occur with any other colour, simple or mixed, and also with white, which unites all the colours. The last in the series is in all cases black. Whence it appears that different parts of the retina are differently sensitive to impressions of colour. The variation occurs in the same order in every direction, but with unequal rapidity. The series is passed through quicker, when the object is moved outwards, than when it is moved inwards; and also quicker for the upward than for the downward movement. It does not follow that in looking at a wide expanse of one colour, we see the gradations of tint in concentric rings. This is only one of many cases where the mind overbears the sense. We have contracted our notion of each surface from the way that its parts affect us when brought successively before the yellow spot-the place of minute examination-and what we seem to see is the habitual effect, rather than the effect at the instant. I shall afterwards allude to an important application of this fact, suggested to explain our power of localizing the different impressions made on the retina. I may advert here also to the phenomena of colour-blindness, and to the suppositions that have been made to account for it. We have already mentioned a speculation, to the effect that the different parts of the ear may respond to different tones or notes. A similar assumption has been extended to the eye. It is considered not unlikely that there are different nerve fibres and endings for the different primary colours, which endings are unequally mixed over the surface of the retina. It may be supposed that at one place violet rods predominate, at another green; and that in the yellow spot the red endings are most abundant. Colour-blindness would then consist in the deficiency or absence of one set of endings. The most frequent form of this defect is obtuseness to the primary sensation of redness; all coloured bodies are then seen as composed of green and violet. The spectrum to such persons is comprehended as of a yellowish and a blueish tinge. What they call white, the ordinary eye sees to be coloured. Colour-blindness has been known to exist with reference to green, but, as yet, not to violet.

experience, and originating a distinct volition. We shall probably meet with no fact attesting more conspicuously the complexity, and yet the separateness of action, of the cerebral system. We can easily satisfy ourselves of the reason why the cerebral hemispheres should be necessary to vision, considering what is thus implied in every instance of seeing whatsoever.

4. On the Adaptation of the Eye to Vision at different Distances. If I see an object distinctly six inches distant from the eye, all objects at a greater distance are indistinct. The image of the near object falls correctly on the retina, the images of remote objects are formed in front of the retina. By a voluntary effort, I can adapt the eye to see a far-off object with tolerable clearness, but it then happens that any near body becomes confused. The questions arise-what is the change produced upon the eyeball, in the course of this adaptation from near to far, and from far to near, and what apparatus effects the change?

In seeing close at hand, the crystalline lens becomes thicker and more convex in front; in seeing at a distance, the surface is flattened. The change of curvature is considerable. The centre-point bulges out th of an inch for near vision. A very slight increase takes place in the curvature of the hinder surface.

The changes of curvature depend on the action of the ciliary muscle. This muscle contracts for near vision; the effect of the contraction is to draw the choroid membrane forwards, and by that means to compress the vitreous humour, which exerts a pressure on the lens, pushing it forwards. At the same time, the muscular fibres of the iris come into play, contracting the pupil and also the outer circumference. This brings a pressure to bear upon the lens from before, but not an equal pressure; it is least at the centre and greatest towards the edges. Between these two pressures, from behind and before, the lens is bulged out in the middle, and its curvature increased. Thus, for near vision, there is a very considerable muscular action; when looking at anything

[ocr errors]
[blocks in formation]

close, we are conscious of a strain in the interior of the ball. For distant vision, this action is relaxed, and the natural elasticity of the parts restores the flattening of the lens. Hence the natural repose of the eye makes the adjustment for a distant prospect.*

The eyeball is subject to alteration chiefly for near distances. Between the smallest visible distance, say four inches, and three feet, nearly the whole range of the adjustment is gone through. When we compare distant objects of varying remoteness, as, for example, thirty feet with one hundred, or a thousand, very little change is effected on the form of the eyeball, the adjustment then depending on the greater or less convergence of the two eyes. This leads to the subject of double vision.

5. Of single Vision with two eyes. Binocular Vision.Among the questions long discussed in connexion with sight, was included the enquiry, why with two eyes do we see objects single? Answers more or less satisfactory were attempted to be given; but since the year 1838, an entirely new turn has been given to the discussion. In that year, Professor Wheatstone gave to the Royal Society his paper on Binocular Vision, wherein he described his 'stereoscope,' or instrument for imitating and illustrating the action of the two eyes in producing single vision. The following quotation is from the opening paragraph :—

'When an object is viewed at so great a distance that the optic axes of both eyes are sensibly parallel when directed towards it, the perspective projections of each, seen by each eye separately, are similar, and the appearance to the two eyes is

* The limits of single vision are illustrated by the following experiment. If a thread is moved against a white wall, and we observe it with one eye through a tube, we can feel a difference when it is moved nearer, but not when it is moved farther away. This is consistent with the circumstances, that in changing to near vision, we cause a muscle to contract, while in changing to a more distant view, the natural elasticity of the parts releases an existing contraction. So, under the same circumstances, we may estimate the interval moved over by the thread, when it is brought nearer; but we can form no estimate of the absolute distance.-(Wundt.)

« ForrigeFortsæt »