strongly upon it. The bright surface of the window will have excited the retina for some time, in consequence of which a definite portion of it will be fatigued, whilst that part upon which the dark window frame was depicted will not be fatigued. If the eye is now turned upon another surface, the fatigued parts will be more feebly excited by this surface than the rest, and consequently a dark window with a bright frame will appear.

A similar experiment may be very well performed in the following manner. A small square of black paper is placed upon white paper, and the eye fixed upon it for some time. If the eye is now suddenly turned upon the white surface of the paper, a bright square is seen upon it which moves about with the eye, and after some time gradually fades away. It is characteristic of this phenomenon that it follows the direction of the eye, and thus proves that we have not to do with a reality, but with an optical illusion, caused by an action within the eye. The most curious part of it is that we, nevertheless, imagine its cause to be external to ourselves, since we have been unconsciously taught so by experience.

Negative incidental images can, however, be perceived with closed eyes. If we close the eyes after looking at a bright object we perceive the same object in a darker tint. This fact seemed for a long time inexplicable, and at variance with the theory of incidental images mentioned above. The instructive investigations, however, of Purkinje, at the commencement of this century, had already shown that, for our eyes, absolute darkness did not exist. In the densest dark

ness, even when, in the darkest night, every trace of light is artificially excluded with the utmost care, the eye has still a perception of light of its own. The sensibility of the eye increases in this darkness in an extraordinary manner, and fantastic clouds of light pass over the field of vision, moving up and down, disappearing and reappearing. It is very probable that this is due to some internal excitement, caused by the circulation of blood in the retina.

This peculiar power of the retina remains even when the eyes are closed, apart from the diffused light which penetrates the eyelids in the day-time, and the negative incidental images which are seen with closed eyes are sufficiently explained by supposing that the fatigued parts of the retina are less sensitive to this feeble sensation of light.

The incidental colours also, which are formed in the eye, are most interesting. It is well known that there are combinations of colour which are pleasant to the eye, and some which are unpleasant or even ugly. Pleasant combinations are blue and yellow, red and green in all shades; while green and blue, yellow and green, and their accompanying shades, are distasteful. We speak therefore of harmonious and unharmonious colours, thus drawing a comparison from music.

If, with reference to this point, we consider the position of colours in the spectrum, we find that harmonious colours are nearly complementary colours, whilst unharmonious colours are situated in the spectrum more or less near to each other,

This does not, however, scientifically explain the cause of the harmonious or unharmonious relation

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. 331 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.'