"My attention has been directed to a paper entitled 'The Theory of Light,' by G. John Smith, Esq., M.A., of Perth Academy. I think it is now nearly two years since I communicated an interesting fact to Professor Faraday, and to a member of our local Philosophical Institution, which may fairly claim to have anticipated Mr. Smith's theory. The fact was this: that if a piece of intensely white card be held in one hand, with the light of a powerful gas-jet falling upon it, and if the other hand has command of the gas-tap, as the light is gradually reduced, the card will assume the prismatic colours down to intense blue, and as the light is restored the colours will present themselves in inverse order. The experiment showed, very conclusively to my mind, that light is homogeneous, and that what we name colour is only the various affection of the optic nerve by a greater or lesser radiation of light from a focal point in an imperfect reflector--say, in the instance, a white card. I apprehend that Mr. Smith confuses his theory when he speaks of alternations of light and shadow producing colour. Shadow, or darkness, is mere negation of light. We do not see mixtures of light and darkness, or blackness and whiteness, but light in its several degrees of intensity. Mr. Smith's experiments present only what my kalotrope has done, and what my later device, the photodrome (now nearly three years old) is doing in a much more perfect manner. It is one of the mysteries intelligible only to the initiated, that whilst Mr. Smith's paper seems to have been received with great favour by the British Association, my communication relative to the photodrome was voted 'not _sufficiently practical_.' * * * * *