Some of the quotations just made direct attention to illusions of Sight which (as we have seen in our last note) Helmholtz elsewhere calls "false inductions." Now one curious fact relative to these impressions is that in many instances the objective consequent is due to a subjective antecedent. Some readers may like to peruse a short account of five variously caused sight-illusions taken from an Oration on Positivism delivered by the present writer at St. George's Hall in May 1871. The particulars here given of the fifth illusion should be compared with the foot-note on page 158 ante.
"I will mention five instances in which people believe they see something, and do not see it; in other words, the objective antecedent is wanting, and the impression is produced partly by the sensory apparatus, partly by the mind itself. As I describe these instances one by one, let my hearers ask themselves, How does this illusion come about? Is it produced by our optic instrument or by our mental activity?
"First, then, Take a lighted stick, and whirl it rapidly round and round. You believe you see a circle of sparks—in reality it is no more than a simple train, and on a like illusion the Catherine-wheel is constructed. Again, put yourself in the hands of an optically inclined friend, and let him operate upon you thus. He shall place a cardboard down the middle axis of your face, quite close against your nose—one side of his board, say the right, coloured a brilliant red, the left a vivid green. After an instant or two let him suddenly substitute another board, white on both sides. Do my young friends guess what will follow? Your right eye will see green, your left red—the reverse of what they saw before; yet neither will see correctly, for both eyes are looking at uncoloured surfaces.
"Thirdly, Watch the full moon rising—how large and round she looks, resting as it were upon that eastern hill, and seen amidst the tops of its forest trees! How much larger and broader than when she hangs aloft in upper sky! Has every one here learned the true reason why? If not, look at her through a slit in a card, and her diameter will be the same.
"Fourthly, A schoolboy is crossing his bedroom in the deep dark night, anxiously hoping that his head may not come into collision with the bed-post. Though carefully and successfully avoiding it, he imagines of a sudden that the blow is imminent. Quick as thought he stops to save his head, and, behold, the room is as quickly filled with sparks or flames of fire. Another moment, and all becomes dark once more. I have heard many a schoolboy exclaim over this phenomenon, but never knew one who could explain it. Finally, did you ever, on opening your eyes in a morning, close them quickly again, and keep them shut, directing them as if to look straight forwards? Most persons of active nervous power, after a few trials—say a dozen, or a score—are surprised to see colours appear and flit before the sight. Some years ago, Germany's greatest poet tried, at the suggestion of her greatest physiologist, a series of experiments on these coloured images. He found that by an effort of will he could cause them to come and go, govern their movement, march, and succession. And this took place under no conditions of impaired sensation, nor any hallucination of a diseased mind. A thoroughly healthy will succeeded in impressing itself upon physical instruments, controlling their law, and creating at its own pleasure an unfailingly bright phantasmagoria.
"Some here may, others may not, have apprehended the distinctions between our five cases. The first two are due to the sensory apparatus, its optical laws of continued impression and complementary colour. In the latter three, mind intervenes. The enlarged size of the moon occurs through rapid comparison, the fiery lights in a dark room through instinctive apprehension, both influences of mind on the sensory system. The fifth and most interesting of all is no bad example of interference between moral and material law. The will truly causative (you may remark) overrules the natural process of physical impression, alters it, and creates a designed effect. I wish I could induce my young friends to devise a number of experiments on similar mixed cases, and, having tried them, to dissect out their real laws. These sharpenings of the critical faculty are exceedingly useful—they cultivate clearness; and most people know that two-thirds among our mistakes in life are caused by confusion of thought.
"Besides all other uses, such lessons teach at once the necessity, as we said before, of observing your own observations. And as, first, the real witness of every observation is our mind; every fact which comes through our bodily senses being to us a mental impression, it seems but common sense to hear above all things what mind has to say for and about itself. Then, secondly, where would be the benefit derived from our observations, if we could not reason upon them, or could place no confidence in our own reasonings? Yet the art of reasoning is so purely a mental process, that it can be represented by symbols as abstract and free from material meaning as if they were bare algebraic signs. Thirdly, in the most accurate of sciences mind extends our knowledge far beyond the circle of observation, and gives us axiomatic assurance of its own accuracy. Who ever saw, or ever can see, all straight lines in all conceivable positions, yet who doubts that throughout the whole universe no two straight lines ever did inclose or can inclose a space? And, fourthly, can it be a matter of indifference to any of us what evidence the mind offers concerning its own moral nature, and what is the value of that evidence, and the laws deducible therefrom? How true it thus appears that 'know thyself' lies at the root of all knowledge, and that the man who receives no witness from within can know nothing as he ought to know it!"