Page:The wonders of optics (1869).djvu/181



illusions of which we have spoken in the first part of this work depended principally on the nature of man's vision, who, we found, was the constant and heed-*less victim of his own powers of sight. We shall now examine a series of illusions that are still more extraordinary, but which have nothing to do with the deceptions practised on us by our visual organs. Instead of being deceived by ourselves, we shall find that we are led astray by others whose knowledge of the laws of optics is greater than our own, enabling them to construct instruments capable of amusing us or imposing on us, according to our ignorance of natural laws. Let us hope, however, that the science of optics has now become so familiar to most educated people, that no such thing as a real imposition can take place, although at the present day there are so many exhibitions of the marvellous that ordinary observers have the greatest