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
