between colours, which is first satisfactorily explained by the following experiment.

If we look for a long time at a green surface and then direct the eye upon a white one, it appears for some moments to be of a red colour. This observation occurs with tolerable frequency in ordinary life. In order to give it a scientific form, lay a small square of green paper upon a sheet of white paper, and look at it very closely for some time with one eye. Then look at the white paper, and a red square is seen upon it, which follows the direction of the eye and gradually fades

away.

Now what is the cause of this incidental colour? The following is the simple explanation: green light does not excite all the elements of the retina which are sensitive to light, but only those sensitive to green, and by looking for a long time at the green paper these have become fatigued. If we then look at the white surface, this excites all the sensitive elements of the retina; the fatigued parts are least excited, so that the complementary colour appears in which the red predominates.

Incidental colours are always seen in complementary colours. If we had been looking at a red paper, then the white surface would look green; the action of blue produces a yellow incidental image, and vice versa. In short, the colour which is seen, and the incidental colour, are always of such a kind, that conjointly, they produce white,

These facts give important support to the theory of colours just mentioned. They explain why red should excite the nerves sensitive to red more readily than the rest, and green and violet the nerves corresponding to

them, and why the excitement of all three together should produce white.

The following is an observation which belongs to this part of the subject and which is frequently made in ordinary life. If we gaze for a moment at the sun, very strong incidental images appear which last for some time. They are always coloured and frequently change their colour. This arises from the fact, that the colours of the incidental image of white sunlight do not disappear simultaneously. When one colour has faded the image is no longer white, the remaining colours appear, which gradually fade away after many variations.

Colours were very early compared to musical sounds, and we have already spoken of the harmony of colours in this sense. This comparison is, however, scientifically satisfactory only to a certain extent. The impressions made by a mixture of colours and a chord in music are very different in character. In a chord a practised ear can hear the different notes, and separate it into its component parts. A mixture of colours, on the contrary, makes an impression as a whole, and can only be separated into simple colours by a practised eye, to a certain It is impossible, however, for the eye to distinguish the primary colours in white, although we may be quite certain that it contains them; while in every combination of tones each tone can be recognised.

extent.

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CHAPTER VII.

The Movements of the Eyes-Binocular Vision-Simple and Double Vision-The Identical Spots of the Retina-The Horopter.

THE eye is endowed with great mobility within its socket. Since it has a spherical shape and the hollow in which it is situated has a spherical shape also, it is evident that this gives the eye the power of turning in every possible direction. The eye, as we know from experience, can be moved with great rapidity, which enables us to direct our attention rapidly to different consecutive objects. We should appear much more clumsy, therefore, if our eyes were fixed firmly in our head, and we were always obliged to move the head from side to side.

Fig. 33' represents the situation of both eyes in their sockets, together with the muscles which give them their power of motion. The socket is enclosed within walls of bone, which, at the back, contract to a funnel-shaped form as far as the aperture through which the optic nerve passes. The socket is filled up with a mass of fat, in which the eye-ball is embedded as in a socket-joint, It encloses nerves, muscles and blood-vessels. The figure Helmholtz, 'Optics.'

Fig. 33.

is called the chiasma. The muscles which are intended for moving the eye, are attached to it like the bridle to a horse's head. They almost all spring from the

also shows the passage of the optic nerve (n) through the bony aperture (o), and, before its exit, the cruciform shape of the combination of both optic nerves (m), which

t

osseous wall at the point where the optic nerve enters, and extend through the entire length of the socket to the eye-ball. There are four optic muscles which pass directly to the eye, of which one is situated above and one below, one on the outer and another on the inner side.

It is clear that the upper muscle directs the eye upwards, making it revolve upon the axis D, the lower one downwards, the inner one inwards, and the outer one outwards. Since, in ordinary vision, we always fix the same point with both eyes, we therefore move them simultaneously according to fixed laws. If we look upwards or downwards with both eyes, then the corresponding muscles are always brought into action. If, on the contrary, we look with both eyes to the right, the outer muscle is brought into action for the right eye, and the inner muscle for the left, and vice versa. If, however, we direct our eye inwards towards a near object, then the two inner optic muscles contract; if the eyes now look at a more distant object, the two outer muscles bring the direction of the eyes more nearly parallel. We are unable to turn both eyes at the same time further apart than when their axes are parallel.

Thus we see that the contractions of optic muscles are connected in many ways. The symmetrical and similarly named muscles frequently contract simultaneously, and the opposite muscles frequently have a common action. All these combinations, however, are intended to enable us to fix the eyes upon the same point, so that the optical axes, drawn from the yellow spot through the centre of the pupil, may meet in the point upon which the eyes are fixed. We are never able so to move the