tung durch Reversionsprismen oder bei umgekehrter Lage des Kopfes schwindet ebenfalls, wenn man beides verbindet. Sie erklärt sich daraus, dass, wenn die richtige Beurtheilung der Ferne schwindet, zu der die Farbenveränderung gehört, der Einfluss der Luft auf die Farben uns in ungewöhnlicher Weise auffällt.

Andrerseits kann man sich auch durch die Betrachtung schwach winklig gebogener Drähte überzeugen, dass man deren Biegungen sehr gut erkennt, wenn sie nahehin in der Horopterlinie liegen, viel schlechter dagegen, wenn sie diese unter einem grossen Winkel schneiden.

Weiter ausgeführt in den Band II dieser Sammlung S. 420 u. 427 als Nr. LXVI und LXVII abgedruckten Abhandlungen.

CVIII.

On the Normal Motions of the Human Eye in relation to Binocular Vision.

Aus: Proceedings of the Royal Society of London. Vol. XIII. (1863-64), p. 186-199. Croonian Lecture. April 14. 1864.

The Motions of the Human Eye are of considerable interest, 186 as well for the physiology of voluntary muscular motion in general, as for the physiology of vision. Therefore I may be allowed to bring before this Society the results of some investigations relating to them, which I have made myself; and I may venture perhaps to hope that they are such as to interest not only physiologists and medical men, but every scientific man who desires to understand the mechanism of the perceptions of our senses.

The eyeball may be considered as a sphere, which can be turned round its centre as a fixed point. Although this description is not absolutely accurate, it is sufficiently so for our present purpose. The eyeball, indeed, is not fixed during its motion by the solid walls of an articular excavation, like the bone of the thigh; but, although it is surrounded at its posterior surface only by soft cellular tissue and fat, it cannot be moved in a perceptible degree forward and backward, because the volume of the cellular tissue, included between the eyeball and the osseous walls of the orbit, cannot be diminished or augmented by forces so feeble as the muscles of the eye are able to exert.

In the interior of the orbit, around the eyeball six muscles

are situated, which can be employed to turn the eye round its centre. Four of them, the so-called recti muscles, are fastened at the hindmost point of the orbit, and go forward to fix themselves to the front part of the eyeball, passing over its widest circumference-or its equator, as we may call it, if we consider the foremost and the hindmost points of the eyeball as its poles. These four recti muscles are from their position severally named superior, inferior, internal, and external. Besides these, there are two oblique muscles, the ends of which come from the anterior margin of the orbit on the side next the nose, and, passing outwards, are attached at that side of the eyeball which is towards the temple—one of them, the superior oblique muscle, being stretched over the upper side of the eyeball, the other, or inferior, going along its under side.

These six muscles can be combined as three pairs of antagonists. The internal and external recti turn the eye round a perpendicular axis, so that its visual line is directed either to the right side or to the left. The superior and inferior recti turn it round a horizontal axis, directed from the upper end of the nose to the temple; so that the superior rectus elevates the visual line, the inferior depresses it. Lastly, the oblique muscles turn the eye round an axis which is directed from 187 its centre to the occiput, so that the superior oblique muscle lowers the visual line, and the inferior raises it; but these last two muscles not only raise and lower the visual line; they produce also a rotation of the eye round the visual line itself, of which we shall have to speak more afterwards.

A solid body, the centre of which is fixed, and which can be turned round three different axes of rotation, can be brought into every possible position consistent with the immobility of its centre. Look, for instance, at the motions of our arm, which are provided for at the shoulder-joint by the gliding of the very accurately spherical upper extremity of the humerus in the corresponding excavation of the scapula. When we stretch out the arm horizontally, we can turn it, first, round a perpendicular axis, moving it forwards and backwards; we can turn it, secondly, round a horizontal axis, raising it and lowering it; and lastly, after having brought it by such motions into

any direction we like, we can turn it round its own longitudinal axis, which goes from the shoulder to the hand; so that even when the place of the hand in space is fixed, there are still certain different positions in which the arm can be turned.

Now let us see how far the motions of the eye can be compared to those of our arm. We can raise and lower the visual line, we can turn it to the left and to the right, we can bring it into every possible direction, throughout a certain range-as far, at least, as the connexions of the eyeball permit. So far the motions of the eye are as free as those of the arm. But when we have chosen any determinate direction of the eye, can we turn the eye round the visual line as an axis, as we can turn the arm round its longitudinal axis?

This is a question the answer to which is connected with a curious peculiarity of our voluntary motions. In a purely mechanical sense, we must answer this question in the affirmative. Yes, there exist muscles by the action of which those rotations round the visual line can be performed. But when we ask, "Can we do it by an act of our will?" we must answer, "No." We can voluntarily turn the visual line into every possible direction, but we cannot voluntarily use the muscles of our eye in such a way as to turn it round the visual line. Whenever the direction of the visual line is fixed, the position of our eye, as far as it depends upon our will, is completely fixed and cannot be altered.

This law was first satisfactorily proved by Professor Donders, of Utrecht, who, in a very ingenious way, controlled the position of the eye by those ocular spectra which remain in the field of vision after the eye had been fixed steadily during some time upon any brightly coloured object. I have used for this purpose a diagram like fig. 1: the ground is grey paper, and in the middle, along the line a b, is placed a narrow strip of red paper on a broader strip of green paper.1) The centre of the red strip is marked by two black points. When you look for about a minute steadily and without moving your eye at

1) Green is represented in the figure by white; red by the central dark stripe.

188 the centre of the diagram, the image of the coloured strips is projected on the nervous membrane of your eye; those parts of this membrane on which the light falls are irritated, and in consequence of this irritation, their irritability is exhausted, they are fatigued and they become less sensitive to that kind

of light by which they were excited before. When you cease, therefore, to look at the coloured strips, and turn your eye either to the grey ground of the diagram, or to any other part of the field of vision which is of a uniform feeble degree of illumination, you will see a spectrum of the coloured strips, exhibiting the same apparent magnitude, but with colours reversed, a narrow green strip being in the middle of a broader red one. The cause of this appearance is, that those parts of your retina which were excited formerly by green light are less affected by the green rays contained in white or whitish light than by rays of the complementary colour, and white light, therefore, appears to them reddish; to those parts of the nervous membrane, on the other hand, which had been fatigued by red light, white light afterwards appears to be greenish. The nervous membrane of the eye in these cases behaves