"On the other hand, the optic nerve, by reason of the peculiar structures in connection with the ends of its fibres, the rods and cones of the retina, is incomparably more sensitive to rays of light than any other nervous apparatus of the body, since the rest can only be affected by rays which are concentrated enough to produce noticeable elevation of temperature. "This explains why the sensations of the optic nerve are for us the ordinary sensible sign of the presence of light in the field of vision, and why we always connect the sensation of light with light itself, even where they are really unconnected. But we must never forget that a survey of all the facts in their natural connection puts it beyond doubt that external light is only _one_ of the exciting causes capable of bringing the optic nerve into functional activity, and therefore that there is no exclusive relation between the sensation of light and light itself." 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.