are light in the other. It is the combination of the two which produces luminosity.

The struggle between the fields of vision, which has been mentioned above, is still more remarkable, if, instead of black and white, we choose two colours, blue and red for instance. We do not then see a single mixed tint, as we might be led to suppose, but a hazy uncertain passage from one colour to the other, which wanders hither and thither. Here and there a mixed tint is seen, and the whole has a somewhat luminous appearance.

The struggle between the fields of vision is also called into action, if two differently coloured glasses are placed before the eyes, when looking at a white surface. The interchange of colours is quite irregular, one colour appearing in the midst of the other without any rule. Some experimenters, however, assert that they see one colour or the other at will, from which it would appear that we are able to give special attention first to the image on the right retina and then to that on the left.

The union of the two retinal pictures into a single picture in relief still remains one of the most wonderful phenomena of the senses, in spite of all these observations, which some regard as quite a sufficient explanation. We must be satisfied for the time in fixing the physical and physiological conditions of these phenomena. The representation of the material world is a distinctly mental act which takes place in the brain, and as such will, for a long time, escape scientific research.

CHAPTER IX.

Optical Illusions-Apparent Size of the Moon-Intuition-Illusions of the Sense of Colour-Mental and Corporeal Vision.

HOWEVER perfect may be the optical apparatus with which nature has provided us, there are cases in which we see things differently to what they are in reality. Such phenomena we call optical illusions, and they are the more interesting because they give us a closer insight into the act of perception.

We can judge with tolerable accuracy as to whether two lines are parallel to each other or not. Zöllner, however, has remarked that we are subject to a curious deception if the parallel lines are crossed by short oblique lines which slant inwards as in fig. 46. The lines 2 and 3 appear at first sight to widen downwards, the lines I and 2 upwards, and yet they are exactly parallel to each other when measured. The oblique strokes on the lines 2 and 3 would intersect each other if extended downwards, and the result is that the lines themselves look as if they would intersect each other if extended upwards. Thus the apparent convergence of the lines always has an opposite direction to that of the oblique strokes.

This phenomenon may be explained as follows:The oblique strokes produce an error of judgment by leading us to imagine that, because they would intersect if produced in an upward direction, the lines must do so if produced in a downward direction. The delusion is stronger if the lines are placed in a horizontal position.

This bewilderment of the judgment in many cases gives an apparent motion to objects. The most familiar example is the giddiness which is felt after we have turned the body round rapidly. For a short time, after the motion has ceased, the objects round us seem to move in an opposite direction. A somewhat similar phenomenon takes place if a number of objects are moved rapidly across a stationary background. In looking, for instance, at a waterfall,

2

Fig. 46.

3

through which the rocks are visible, we are, after a time, seized with the impression that the rocks are moving upwards.

In these cases we transfer some of the rapidity of the object in motion to the object which is at rest, but in the opposite direction. This phenomenon is still more striking if, when sitting in a train which is not in motion, we look at one which is passing. We are very

frequently impressed with the idea that we are moving ourselves, and the other train is standing still, and we can only undeceive ourselves by looking at the motionless objects through the other window. In this case we transfer all the rapidity of the other train to ourselves, and thus receive the impression of motion.

It seems as if the motion of the eyes played a considerable part in many of these illusions. When looking at Zöllner's figure, our eyes may easily be displaced from their ordinary position, by rotation round their axes, into an oblique position, from looking at the oblique lines, so that the vertical planes of the optical axes are at an oblique angle to each other instead of being parallel, and this may contribute towards the error in our judgment as to the direction of the parallel lines. In support of this view Helmholtz mentions the fact that, in the momentary illumination of the electric light, the illusion is absent or at least much weaker, because in the moment of vision there is not sufficient time for the eyes to make any movement.

The movement of the eyes can also assist in producing giddiness. For whilst we are turning round, the eye tries to fix for a time every object seen, and in our revolution passes in a backward direction to the next object. In this manner a backward motion of the eyes is produced which will continue for some seconds after our own motion has ceased, and it is probably this which causes the apparent motion of objects. At least all the motions of the eyeball, which are made by the muscles of our eyes, independently of our consciousness or will, giv ise to an apparent motion in the surrounding

This is the case in the rapid motion of the

eyes in illness, which produces a feeling of giddiness. This apparent motion of objects is observed if we displace the eye-ball with the finger, and move it gently backwards and forwards. Objects will then appear to move in an opposite direction to the motion of the eye-ball, because we are unconscious of the motion. of the eye-ball in the reverse direction.

A somewhat similar effect is produced by looking for some time at objects in motion. In looking at a waterfall the eye endeavours to follow the falling mass for a certain distance, and from continually returning to the first point, acquires a backward motion. If we continue looking for some time, we shall notice that the stationary objects round us appear to be moving upwards because we are unconscious of this downward motion of the eye.

This confusion of our judgment on horizontal, upward and downward directions is of frequent occurrence.

Let

a.

Fig. 47.

us suppose, for instance, that when walking down a gentle incline bon a road (fig. 47), we see at some distance before us a road a, which has exactly the same inclination, and which joins b. We shall find that w