The cause of colour-blindness is completely unknown; philosophers and physiologists are still in the realms of hypothesis concerning this peculiar optical defect. As yet, the most careful observation has failed to detect any difference between the eyes of those who are colour-blind, and the eyes of ordinary persons, that could in any way account for this singular affection of the sense of sight. ------------------------------------------------------------------------ CHAPTER VI. ILLUSIONS CAUSED BY LIGHT ITSELF. WHEN playing about the Christmas fire, children frequently amuse themselves by whirling round and round a piece of wood, one end of which they have previously lighted and blown out. In proportion as the movement becomes more rapid, the path of the red-hot end becomes more and more connected, until at last a burning ring is formed, in every part of which the shining charcoal appears to be at the same time. The only way of accounting for this illusion is by supposing that the image formed by the burning stick upon the retina remains there for an appreciable period, the impression made by it at one part of its journey remaining until it returns to its former position. The power possessed by the retina of retaining impressions explains a large number of illusions of the same kind. The chord of a musical instrument, for instance, when struck, appears to occupy a longer space during the time it vibrates, than when it is at rest. A rapidly revolving wheel appears almost solid on account of the combined images of the spokes seeming to unite into one homogeneous mass. The persistence of luminous impressions upon the retina has given rise to the invention of a number of well-known optical toys, amongst which may be mentioned the _phenakistiscope_, the _thaumatrope_, the _phantascope_, and many others. [Illustration: FIG. 3.—The Phenakistiscope. ]