ABSTRACT
The apparitional and unsubstantial character of the image is derived from its seeming transparency, and from the manner in which it may be made to melt away, by diminishing the brightness of the light which falls on the real person. Professor Tyndall, in the course of some remarkable researches on the decomposition of vapours by light, wished to have such a glass tube, filled with air perfectly free from floating particles. The apparitional and unsubstantial character of the image is derived from its seeming transparency by diminishing the brightness of the light which falls on the real person. Huyghens studied the subject afresh, and was able, by a geometrical conception, to bring the new phenomena within the general theory of light. The ray of light is no longer uniform in its properties all round, but appears to have acquired different sides, as it were, in passing through the rhomb of Iceland spar.
