gives white, while anabolism, in the dark, gives rise to the sensation
of blackness. Thus blackness is a sensation as well as whiteness,
and the members of each pair are antagonistic as well as
complementary. In the red end of the spectrum the rays cause katabolism
of the red-green substance, while they have no effect on the yellow-blue
substance. Here the sensation is red. The shorter waves
of the spectral yellow cause katabolism of the yellow-blue material,
while katabolism and anabolism of the red-green substance are here
equal. Here the sensation is yellow. Still shorter waves,
corresponding to green, now cause anabolism of the red-green substance,
while their influence on the yellow-blue substance, being equal in
amount as regards katabolism and anabolism, is neutral. Here
the sensation is green. Short waves of the blue of the spectrum
cause anabolism of the yellow-blue material, and as their action on
the red-green matter is neutral, the sensation is blue. The very
short waves at the blue end of the spectrum excite katabolism of the
red-green substance, and thus give violet by adding red to blue.
The sensation orange is experienced when there is excess of
katabolism, and greenish blue when there is excess of anabolism in both
substances. Again, when all the rays of the spectrum fall on the
retina, katabolism and anabolism in the red-green and yellow-blue
matters are equal and neutralize each other, but katabolism is great
in the white-black substance, and we call the sensation white.
Lastly, when no light falls on the retina, anabolic changes are going
on and there is the sensation of black.
Hering's theory accounts satisfactorily for the formation of coloured after-images. Thus, if we suppose the retina to be stimulated by red light, katabolism takes place, and if the effect continues after withdrawal of the red stimulus, we have a positive after-image. Then anabolic changes occur under the influence of nutrition, and the effect is assisted by the anabolic effect of shorter wave-lengths, with the result that the negative after-image, green, is perceived. Perhaps the distinctive feature of Hering's theory is that white is an independent sensation, and not the secondary result of a mixture of primary sensations, as held by the Young-Helmholtz view. The greatest difficulty in the way of the acceptance of Hering's theory is with reference to the sensation of black. Black is held to be due to anabolic changes occurring in the white-black substance. Suppose that anabolism and katabolism of the white-black substance are in equilibrium, unaccompanied by stimulation of either the red-green or the yellow-blue substances, we find that we have a sensation of darkness, but not one of intense blackness. This “darkness” has still a certain amount of luminosity, and it has been termed the “intrinsic light” of the retina. Sensations of black differing from this darkness may be readily experienced, as when we expose the retina to bright sunshine for a few moments and then close the eye. We then have a sensation of intense blackness, which soon, however, is succeeded by the darkness of the “intrinsic light.” The various degrees of blackness, if it is truly a sensation, are small compared with the degrees in the intensity of whiteness. In the consideration of both theories changes in the cerebral centres have not been taken into account, and of these we know next to nothing.
6. The Contrast of Colours.—If we look at a small white, grey or black object on a coloured ground, the object appears to have the colour complementary to the ground. Thus a circle of grey paper on a red ground appears to be of a greenish-blue colour, whilst on a blue ground it will appear pink. This effect is heightened if we place over the paper a thin sheet of tissue paper; but it disappears at once if we place a black ring or border round the grey paper. Again, if we place two complementary colours side by side, both appear to be increased in intensity. Various theories have been advanced to explain these facts. Helmholtz was of opinion that the phenomena consist rather in modifications of judgment than in different sensory impressions; J. A. F. Plateau, on the other hand, attempted to explain them by the theory of consecutive images.
5. The Movements of the Eye
1. General Statement.—The globe of the eye has a centre of rotation, which is not exactly in the centre of the optic axis, but a little behind it. On this centre it may move round axes of rotation, of which there are three—an antero-posterior, a vertical and a transverse. In normal vision, the two eyes are always placed in such a manner as to be fixed on one point, called the fixed point or the point of regard. A line passing from the centre of rotation to the point of regard is called the line of regard. The two lines of regard form an angle at the point of regard, and the base is formed by a line passing from the one centre of rotation to the other. A plane passing through both lines of regard is called the plane of regard. With these definitions, we can now describe the movements of the eyeball, which are of three kinds: (1) First position. The head is erect, and the line of regard is directed towards the distant horizon. (2) Second position. This indicates all the movements round the transverse and horizontal axes. When the eye rotates round the first, the line of regard is displaced above or below, and makes with a line indicating its former position an angle termed by Helmholtz the angle of vertical displacement, or the ascensional angle; and when it rotates round the vertical axis, the line of regard is displaced from side to side, forming with the median plane of the eye an angle called the angle of lateral displacement. (3) Third order of positions. This includes all those which the globe may assume in performing a rotatory movement along with lateral or vertical displacements. This movement of rotation is measured by the angle which the plane of regard makes with the transverse plane, an angle termed the angle of rotation or of torsion.
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Fig. 23.—Diagram of the Attachments of the Muscles of the Eye and of their Axes of Rotation, the latter being shown by dotted lines. (Fick.) The axis of rotation of the rectus internus and rectus externus being vertical, that is, perpendicular to the plane of the paper, cannot be shown. |
The two eyes move together as a system, so that we direct the two lines of regard to the same point in space.
The eyeball is moved by six muscles, which are described in the article Eye (Anatomy). The relative attachments and the axes of rotation are shown in fig. 23.
The term visual field is given to the area intercepted by the extreme visual lines which pass through the centre of the pupil, the amount of dilatation of which determines its size. It follows the movements of the eye, and is displaced with it. Each point in the visual field has a corresponding point on the retina, but the portion, as already explained, which secures our attention is that falling on the yellow spot.
2. Simple Vision with Two Eyes.—When we look at an object
with both eyes, having the optic axes parallel, its image falls
Fig. 24.—Diagram
to illustrate the
Physiological
Relations of the two
Retinae.
upon the two yellow spots, and it is seen
as one object. If, however, we displace
one eyeball by pressing it with the finger,
then the image in the displaced eye does
not fall on the yellow spot, and we see
two objects, one of them being less
distinct than the other. It is not necessary,
however, in order to see a single object
with two eyes that the two images fall
on the two yellow spots; an object is
always single if its image fall on
corresponding points in the two eyes.
The eye may rotate round three possible axes, a vertical, horizontal and antero-posterior. These movements are effected by four straight muscles and two oblique. The four straight muscles arise from the back of the orbit, and pass forward to be inserted into the front part of the eyeball, or its equator, if we regard the anterior and posterior ends of the globe as the poles. The two obliques (one originating at the back of the orbit) come, as it were, from the nasal side—the one goes above the eyeball, the other below, while both are inserted into the eyeball on the temporal side, the superior oblique above and the inferior oblique below. The six muscles work in pairs. The internal and external recti turn the eye round the vertical axis,
