There are some illusions which arise from certain of the laws of action of impressions on the retina—that tissue of the eye in which the changes necessary to the excitation of the sensation of light by luminous rays are induced.
A sensation excited in the retina is not momentary, or during the continuance of the exciting cause alone, but it persists some seconds after that has been withdrawn. Thus if the end of a burning stick be rapidly moved in a circle before the eyes, it gives rise to the sensation of an uninterrupted circle of light; the sensation excited on each part of the retina enduring for a certain period after the luminous point has passed.
The following instance is an example of an illusion, having relation to our subject, from this cause.
A gentleman had been earnestly regarding a small and very beautiful painting of the Virgin and Child. On turning round from the contemplation of it, he was surprised at finding a woman of the full size, with an infant in her arms, standing before him. On examining the figures more closely he, however, found that the woman wanted the lower fourth of the body, and this at once led to a correct appreciation of the nature of the phantom. The painting he had been viewing was a three-parts length, and it was the persistence of the image upon the retina for a short period after he had turned from it, which had given rise to the phantom.
A species of divination is made use of in India which has its origin in an illusion of this nature, and of which the following is an interesting example:—
A lady who was about to undertake a long journey, was persuaded by a Moonshee to walk on the verandah and consult her fate.
"It was a clear calm night, the moon was full, and not the faintest speck in the sky disturbed her reign. The Ganges was like a flood of silver light, hastening on in charmed silence; while on the green smooth sward on which they walked a tall shrub here and there stood erect and motionless. The young lady, whose impressions were probably deepened by the mystical words of the Moonshee, felt a kind of awe stealing over her; she looked round upon the accustomed scene as if in some new and strange world; and when the old man motioned her to stop, as they reached an open space on the sward, she obeyed with an indescribable thrill.
"'Look there,' said he, pointing to her shadow, which fell tall and dark upon the grass. 'Do you see it?'
"'Yes,' said she faintly, yet beginning to be ashamed. 'How sharply defined are its edges! It looks like something you could touch!'
"'But look longer, look better, look steadfastly. Is it still definite?'