# The wonders of optics/Optical illusions

Layout 2
OPTICAL ILLUSIONS.
Besides the errors of sight already spoken of, there are other illusions, which are either common to all persons or confined to certain individuals, the knowledge of which will serve as a fitting prelude to the descriptions of those which are artificial.
The following defect, for instance, is one which is little known, but notwithstanding our ignorance of its existence it is nevertheless true that we all suffer from it. There is in every one's eye a blind spot, totally incapable of experiencing the effects of the rays of light when they impinge upon it. For objects situated opposite to this particular spot we are as completely blind as if we had no eyes at all. To convince yourself of the truth of this assertion it is only necessary to try the following simple experiment.
Place upon a piece of white paper two small wafers, or two blots of ink about an inch and a half apart. Take the sheet in your right hand, and hold it up parallel to the lines of the eyes; shut the left eye, and fix the right eye on the centre of the left wafer or ink-spot. Move the sheet of paper steadily towards the eye, until it is about two inches and a half or three inches' distance from it, and you will find that in a certain position the other wafer or ink-spot will disappear, although it is evidently still in the field of view. Having discovered ​this point which differs for different eyes, you will find that if you diminish or increase the distance of the paper you will once more see the missing object. The same thing happens if you move the eye from the centre of the wafer. The same experiment may be repeated with the left eye with a precisely similar result.
It has been found by experiment that this particular blind space exists exactly over the base of the optic nerve, at the spot where it joins the eye. (Fig. 1.) Thus we see that the nerve which actually conveys the impression of sight to the brain is in itself incapable of being excited by light. In such cases as these Nature seems to laugh at us, and escapes from our grasp just as we are most confident in our power of wresting her secrets from her; indeed we may compare her to a wise and good-natured mother, who, though always amiable and willing to instruct those about her, sometimes smiles when her children fancy they are as learned as she is.
If we do not perceive the constant recurrence of the phenomenon just mentioned, it is because when both eyes are open the object whose image falls on the blind spot in one eye is seen by the other, the insensible portions of each eye being on opposite sides. Not only this: the spot being always situated on the outer and indistinct portion of the image reflected on the retina, we do not take notice of it; for as every one has no doubt observed, it is only the small portion of the object we are looking at exactly opposite the centre of the eye that is perfectly distinct and clear, all the rest being confused in its details, although quite visible.
Again, we may account for our not noticing it by the fact of our seeing clearly only those things which specially attract our attention—a fact first noticed by Mariotte. We see only what we wish to see with our physical eyes, as well as those of our mind. If our ​attention is attracted by a particular portion of a landscape, we see only that, and nothing else. If it is fixed on some subject that we are contemplating inwardly, we see nothing at all, although our eyes may not only be wide open, but absolutely fixed on some particular object. For instance, suppose a sportsman is out in the fields preceded by his dogs, Bran and Ponto. If he follows the movements of Bran with attention, he becomes the only object animate or inanimate, that depicts itself on his retina. Ponto may jump and caper in vain: he is lost to his master's eye as much as if he were not there at all; his mind is entirely fixed on the beauty of Bran's coat, on the fit of his collar, or fifty other things, and he sees nothing else. But let the sportsman begin to think of the number of birds he shot yesterday, or how he will find time to get up to the grouse in Scotland, or of that fine stag he missed when he was last amongst the heather, and dogs, cover, and landscape will fade from his sight as effectually as if he had been struck with blindness. Let him, however, strike his foot against a stump, or let the dogs suddenly begin to point, and he instantly receives back his sight, which but a few moments before he had lost to all intents and purposes.
The phenomena of ocular spectra and complementary colours experienced by every one forms a curious chapter in the history of those illusions which take their origin in the eye itself. Every one has noticed that after looking fixedly at a bright light or a striking colour for a few moments, the eye preserves an impression of the object for a certain time. A very light window looked at intently for several seconds will leave the impression of its cross-bars on the retina for several minutes, the colour of the image changing at every movement of the eye. The same effect may be observed ​when looking at the setting sun, or a flaring gas light. If the light at which we look is coloured, we shall see the complementary colour in the impression left on the retina. Sir David Brewster was one of the first to notice and experiment upon these very interesting facts.
If we cut out any simple figure, a small cross for instance, in scarlet paper, place it upon a white background and look at it fixedly for a minute or two, we shall find that its tint will gradually become duller. If we now suddenly look at a piece of white paper, we shall see the cross depicted upon it in green, which is the complementary colour to red. It should be explained, that the complementary of any colour is that which is necessary to make white light. Thus, blue, yellow, and red (as we shall find out when we come to speak of the prismatic spectrum), mixed in certain proportions, form white light; consequently the complementary of orange, which is composed of red and yellow, will be blue; of green, which is yellow and blue, red; of purple, which is blue and red, yellow, and vice versâ. The complementary of black is white, and of white, black as a rule; but if the white object be very brilliant, the black spectrum will speedily become coloured. The impression left by the setting sun is of this character. At first, while the eye is open, the image is black, then brownish red, with a light blue border; but if the eye be shut suddenly, it becomes green, with a red border, the brilliancy of colour being apparently in proportion to the strength of the impression. These spectra may be perceived for a long time, if the eye is gently rubbed with the finger now and then. Some eyes are more impressionable in this respect than others, and Beyle gives an instance of an individual who saw the spectrum of the sun for years, whenever he looked at a bright object. A modern instance of ​this occurred lately to an amateur astronomer who was looking at an eclipse of the sun. He unfortunately used a glass that was not sufficiently smoked, and the image of the sun's disc, with the black space caused by the intervening moon, remained on his retina for months after. This gentleman's case afforded an instance of the necessity of attention in order to see any object, for after the first few days he only became sensible of his unfortunate mishap when his attention was called to it by some accidental circumstance. These facts were so inexplicable to Locke, that he consulted Newton on the subject, and was surprised to learn that the great philosopher himself had suffered for several months from a sun-spectrum in the eye.
Without affirming that optical illusions are the cause of all the supposed supernatural appearances of which we have heard so much, there is no doubt that in many instances the eye plays an important part in deluding the brain. The following example, also cited by Beyle, will show this clearly. A horseman dressed in black, and riding a white horse, was trotting along a portion of the road, which through a sudden break in the clouds was brilliantly illuminated by the rays of the sun. The black figure of the man was projected against a white cloud, and the horse appeared doubly brilliant from being seen against the dark-coloured road. A person who was greatly interested in the arrival of the horseman was watching them with great attention, when suddenly the horse and his rider disappeared behind a wood. An instant after the observer was terrified at seeing a white cavalier on a black horse projected on a white cloud at which he was accidentally looking. It may be readily imagined that such an occurrence, followed up by a succession of unusual events,—such as illness, death, or any other series of misfortunes,—might even in the present day add a chapter to the history of the marvellous.
​To the illusions to which, like the preceding, we are all subject, may be added those resulting from some abnormal conformation, or some disease of the eye, in those who labour under them. An example of this occurs in the case of double or triple vision, many remarkable instances of which are mentioned by Müller, the celebrated physiologist.
Although, as before explained, the image of an object is depicted at the same time on both our eyes, still we only see one impression, in consequence of the two images being carried to the brain from corresponding portions of the retina. If this relation be disturbed by any cause, or if the eyes are not converged exactly upon the same point, a double image is the result. The first of these facts may be proved by looking at the moon, for instance, with the left eye shut; on suddenly opening it, two images will be seen for an instant. The second is instantly proved by pushing either of the eyes aside with the finger, when looking at any object.
It is necessary, however, to distinguish between these effects and true double vision, as well as a certain defect which exists in the eyes of many people, consisting in the apparent multiplication of distant objects by the same eye. In these cases, there is a superposition of images upon the retina, each having its proper bounds. With the majority of individuals afflicted in this way, it only happens when they look at a very distant object, the moon or stars for instance. There are many, however, who suffer from it in the case of everything they look at, whether far or near. Stephenson, who was affected with it, made it the subject of many interesting experiments. When he looked at a clear mark on a white ground, and gradually walked away from it, not only did the image become indistinct, but it seemed to unfold itself into several, independently of many others much ​more indistinct, more especially two situated on each side, whose distance increased the farther he walked away. As these latter images became more and more separated, they also became more confused. The image seen by the right eye was a little higher than that seen by the left. Griffin states, that after having used the telescope for any length of time, the eye that he kept shut always saw objects triple and double for some hours afterwards. These phenomena are possibly connected in some way with the disposition of the plates and fibres of which the crystalline lens of the eye is composed.
Semi-vision, or hemiopia as it is called, is much more rare and more difficult to explain than the phenomena of double vision; and consists in the power of being able to see only the right or left half of the object looked at, the separation being vertical when the eyes of the observer are in the same horizontal line. Thus, in looking at the word Newton, the person so afflicted would only see either the letters New or TON according to which half of the eyes were defective.
Wollaston was afflicted with hemiopia on two different occasions; the first time after violent exercise, during two or three hours, when he could see distinctly only the left-hand halves of the objects he was looking at. Both eyes were similarly affected, and the phenomenon only lasted about a quarter of an hour. Twenty years afterwards he suffered again from the same accident, but on this occasion in the contrary manner; that is to say, he only saw the right halves of the objects he was looking at—to use his own words, he could only see the right half of every friend he met. At certain distances from the eye, one of two persons would become invisible, and by simply changing his own position or that of the persons he was near, he could make one or other of them, or indeed both, ​disappear at will. It must be acknowledged that similar tricks of Dame Nature, due to an unconscious insensibility of the eye, are most singular, and at first sight appear to have a supernatural origin.
Bartholin mentions the case of a hysterical woman who was afflicted with hemiopia horizontally, and saw all natural objects cut in two, the lower halves being invisible. In this instance it was only the left eye that was defective.
Another interesting example of optical illusion is the luminous sensation produced internally when the eye, or the neighbouring parts, are struck or stimulated by friction or electricity. These appearances are experienced even by those who have lost their sight. Müller states that a case was submitted to a legal tribunal to decide whether the luminous sensations which are perceptible when we rub our eyes are really light. The matter in dispute was whether a man who was attacked by robbers in the dark, could see and recognise them by means of the light produced in his eyes by a violent blow on the head; but he does not tell us how the question was decided. With regard to internal causes, Humboldt tells us that a man whose eye had been extirpated, was sensible of luminous appearances whenever he was galvanized. Lincke states that a man whose eye had been removed by a surgical operation, saw next day all kinds of luminous phenomena, which tormented him cruelly with the idea that after all his eye had been saved. When he shut the perfect eye, he fancied he saw with the missing eye circles of fire, persons dancing, and similar appearances for several days. These facts are analogous to those told of persons who have had their legs and arms amputated, but who, notwithstanding, apparently feel pain in their lost limbs.