such representations of solid objects, the boy would often ask those around him which of his senses was deceiving him, his sight or his touch. At first he saw everything of an enormous size, but as he saw things larger than those around him, he found the latter diminish. He also imagined that there was nothing beyond the room he was in, and could not be brought to comprehend how the house could be larger. When the sight of the second eye was restored to him a year afterwards, he at first saw every object of an enormous size, just as in the case of the first eye; but as he had now the perfectly educated organ to help him as well as his sense of touch, he soon began to see things under their natural appearances. While he was in ignorance of what sight really meant, he was not particularly anxious to undergo the operation, saying that he did not think it possible to derive more pleasure from things that he liked than he did while he was blind. But now that his sight was restored he found every fresh object a new pleasure. When first he was shown the landscape from the top of a high hill, he was so delighted that he exclaimed that he had found another sense. When his second eye was operated upon, he saw things apparently twice as large with both eyes as with the one already restored to him. Even at first he seemed to have no difficulty in converging the eyes on any object. These extracts from the history of Cheselden’s patient show us how utterly incapable the eye must be of rightly understanding the number, position, size, and form of objects without frequently correcting our impressions by the aid of the sense of touch. ------------------------------------------------------------------------ CHAPTER IV. 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.