CHAPTER VI.

Incidental Images-The Phantascope-Positive and Negative Incidental Images-Incidental Colours-Harmony of Colours.

IN ordinary vision a picture disappears as soon as the object seen is withdrawn, or it has ceased to be illuminated. In some well-known phenomena, however, it may be observed that the impression of light lasts for an appreciable time after the light is withdrawn. If a burning stick is whirled quickly round in a circle, we have the impression, not of a point, but of a fiery circle. Similarly, a rising rocket and a falling star produce the impression of a line of light. Moreover, if, in the night, the darkness is illuminated by a flash of lightning, we always have the impression that the flash has lasted for some time; although, in reality, it is of such momentary duration, that if a railway train, as it rushes by, were illuminated by lightning it would appear to be standing still. The persistence of these impressions is unpleasant if we have looked at a powerful light or the sun. We then perceive spots of light for some time, even when the eyes are closed, which greatly inconvenience our sight.

These prolonged impressions of light are called

incidental images. These images may be observed of all bodies, which are not too feebly illuminated, if the eye is sufficiently sensitive; especially of a bright window, when the eyes are opened and closed rapidly. After a few seconds a faint image of a window is observed, which gradually disappears. The existence of incidental images has already been pre-supposed in the mention of the rotating disc, which contained black and white, or coloured sections. If the rotation is sufficiently rapid we no longer perceive the single sections, but a combination of their colours. We have here to do with the phenomenon of incidental images, for the image of the black section is superposed upon the image of the white one with such rapidity, that the incidental image of the black section has not yet. disappeared; the images, therefore, of the black and white sections are mentally conbined, and the result is a grey. The revolutions must, therefore, be made with a certain velocity, for the grey colour appears when white follows black about thirty times in a second. If the revolution is less rapid it produces a flickering between the black and white, which produces an unpleasant and fatiguing impression upon the eye. This sensation is still more troublesome with a flickering light, since as the light diminishes the eye each time has a short rest, and is irritated with increased vigour by the light when it breaks out again.

The persistence of these incidental images is the basis of a well-known instrument, the phantascope, or magic disc, on which various figures are seen in motion, such as dancers, horsemen, gymnasts, etc. Fig. 32 explains this phenomenon by a simple example.

Upon

a disc a pendulum is represented in different positions of its oscillation. If we look, from the other side, through the holes 1-12 upon a mirror, and turn the disc with

Fig. 32.

tolerable rapidity, the pendulum is seen first in the position of I, then in 2, 3, etc.

The

images follow each other very rapidly, so that the pendulum appears to have made an oscillation. Here the incidental image of one pendulum remains a sufficient length of time for the image of the next to take its place.

From the kind of incidental images we have just considered, must be distinguished those which are formed when we look for a long time at a bright object. Look, for instance, through a window at the bright sky for from half to a whole minute, without moving the eyes, so that the window frame may be depicted upon a definite part of the retina, and then turn the eyes quickly towards a wall, or a white sheet of paper. A dark window will now be seen with a bright frame, so that in this incidental image all the bright parts of the picture appear dark, and the dark bright. On this account it is called a negative incidental image, while the former kind of incidental image is called positive.

Negative incidental images are caused by the retina being fatigued at the spot where the light has acted

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