A DOPTING the chronological order in which the radia-tions of radium and other sources were discovered and applied, the lecturer considered, in the first place, the effects of light and radiant heat, dwelling especially upon the fact that the chemical rays—i.e. blue, violet, and ultraviolet—were those which had vital effects upon the tissues. The differential effect of these rays as compared with those of longer wave-length at the other end of the spectrum was well brought out in the course of an experiment in which, the spectrum having been produced upon the screen, a strip of bromide paper was stretched across it so as to receive the length of the spectrum, and this on being developed and fixed was shown to have darkened very considerably at the blue and violent portion, while the red end of the spectrum was practically white paper. The more ready absorption of these blue and violet rays, known to everyone who has looked at the reddened disc of the sun through a somewhat dense fog, was further illustrated by placing a glass cell in front of the lantern and filling it with hypo solution to which some hydrochloric acid was added. Ultimately, only the red waves were able to penetrate. The lecturer further pointed out that the blue colour was the last to be seen at the close of day; that when a person had his sight temporarily impaired by over-indulgence in tobacco he lost the perception of red and green in the centre of his field of vision, but very rarely lost the perception of blue; and that in cases of blindness coming on gradually from wasting of the optic nerve, blue was the last colour to go.