Such being the case, it needs no argument to show the importance of making quite sure that our interpretation of nature is correct. If there be any unobserved illusion in our sensory instruments, or what must evidently be much worse, in our percipient mind, truth is at an end, and falsehood received in its stead. Hence the necessity of observing our own observations, subjecting our consciousness to scrutiny, and being acquainted with the criteria, not only of our perceptions, but of our judgments. It is this process of analysis and criticism which forms a large part of the method of verification,--a method the value of which did not escape the great Greek philosophers, though some recent writers seem to fancy it a modern discovery. Inexperienced observers are often so little aware of the pre-eminent importance of this critical process, that I will detain you with an illustration of it for the benefit of my younger auditors. My example shall be taken from perception _par excellence_--our eyesight, _the_ sense pronounced surest both in poetry and prose. You will remember your Horace Segnius irritant animos demissa per aurem, Quam quæ sunt oculis subjecta fidelibus, et quæ Ipse sibi tradit spectator. And almost everybody else has said the same, as witness the old proverb, "Seeing is believing." Now 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.