EYE 825 image; (4) if we regard a red surface, the primitive colour, the red fibres are little ati ected in consequence of being fatigued, the green and violet fibres will be only feebly excited, and therefore only a very feeble complementary image will be seen; and, (5) if we look at a surface of a different colour altogether, this colour may combine with that of the consecutive image, and produce a mixed colour; thus, on a yellow surface, we will see an image of an orange colour. (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 circls 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 is of opinion that the phenomena consist more in modifications in judgment than in modifications of sensa tion; Plateau, on the other hand, attempts 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, (I) First position. The head is erect, and the line of regard is directed towards the distant horizon. (2) Second position. This indi cates all the move ments round the transverse and hori zontal 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 T l "f displacement, or the FIG. 23. Diagram of the attachments of the ascensional angle; muscles of the eye and of their axes of rota tion, the latter being shown by dotted lines. (Fick.) The axis of rotation of the rectus internus and re- externus being vertical, that is, perpendicular to isplacedfrOIll " le P ane f the paper, cannot be shown. 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 rota tion is measured by the angle which the plane of regard i trit and when it rotates round the vertical the line of makes with the transverse plane, an angle termed the angle of rotation or of torsion. 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 ANATOMY, vol. i., p. 891. The relative attach ments and the axes of rotation are shown in fig. 23. The following table, given by Beaunis, summarizes their action : Number of Muscles in activity. Diiection of Line of Regard. Muscles acting. One .(Inwards (Internal rectus. (External rectus. (Superior rectus. (Infeiior oblique. (Inferior rectus. (Superior oblique. (Internal rectus. < Superior rectus. (inferior oblique. (Internal rectus. < Inferior rectus. (Superior oblique. (External rectus. -j Superior rectus. (inferior oblique. ( External rectus. < Inferior rectus. (Superior oblique. Two ( OutwaiQs
Upwards
I Downwards Three Inwards and up wards Inwards and down- Outwards and up- Outwards and downwards 6 of the two 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. W T hen we look at an object with, both eyes, having the optic axes parallel, its image falls upon the two yellow spots, t and it is seen as one object. If, how- / ever, 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 FlG 2 4. Diagram single object with two eyes that the two to illustrate the images fall on the two yellow spots; an physiological rela- object is always single if its image fall on corresponding ywints in the two eyes. Thus, in the experiment above described, after having seen two images by displacing one eyeball, we may be able again to see only one image t by pressing on the other / eyeball. There are then cor- / responding points in the two / retinae, so that if they were superposed the two yellow spots would coincide; the upper and lower parts of the left retina would touch the upper and lower parts of the right retina; the nasal side of the left retina would correspond to the temporal side of the right retina, and the reverse would also hold good. The relation of the 3 two retinae to each other in. the field of vision may be 5 6 illustrated by the diagram FlG - 25. Diagram to illustrate phe- infig. 24. When an image nom ena of double vision. (Miiller.) falls on non-corresponding points of the retina, it is seen double. VIII.
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