Our theory of the constitution of light, upon whicli we a terpret the phenomena of inherent and transient colours by chemical election, is fully adequate to the explanation of the colours of transmitted and reflected light. Thus, when light passes through transparent coloured glass, it is not the colours the glass that tinges the light so transmitted, in the manner colouring substance tinges a liquid, but the colour in the glow neutralizes itself, or retains from the light by election such a proportion of its principles as reduces such colour to an achromatic state, and suffers the remainder of the light to pe through the glass, so constituted as to afford the precise colour of the glass itself. In this manner such a proportion of the principles of light as constitutes redness passes through red glass and the rest of the light is retained. It is the same with other colours of transparent substances; and, in like manner, the colours reflected from opaque coloured bodies are not the co-lours of the bodies themselves, but of the light by which they are illuminated; nor do we in any case see the immediately colours of objects, hut those only with which they affect light. Nevertheless, some substances have such chemical relations with active light and its opposite as to reflect one colour, and transnut another, which, id general, are complementary; but, in some cases, such substance fixes a portion of the principles of light in such a manner that the colour transmitted is not complementary or equivalent to that reflected. Of the first, chloride of sulphur is an example, being red by reflected light, and green by transmitted; again, the filaments of selenium are orange-rul by reflected light, but blue-gray by transmitted; but gold-leaf, which reflects yellow; transmits green, and fixes purple-red.

Upon the abstraction or separation of unequal portions of the two elements of light, we arc disposed to regard nil the phenomena of the polarization of tight as dependant, and chiefly so upon the absence of the oxygenous or active principle, whereby light is thrown into an electrical or chemicaJ state. In this opinion we are strengthened by the disposition of polarized light to develope colours, by means of the tourmaline, from crystallized plates of mica, nitre, etc., all the phenomena of which are conformable to chromatic relations, as deduced from light, shade, and contrast. In fine, we regard these two principles as positive and negative in electricity and galvanism; polar, in magnetism and light; active and reactive in all physics, and elementary and identical throughout nature. As yellow fixes or absorbs one proportion of the elementary principles of light - red another - blue another - and block absorbs the whole of light, so coloured bodies arc found to become heated by the sun's rays in proportion as their colours retain or fix light, or refuse its transmission or reflection.

That colours and light itself are oxide* of hydrogen is a doctrine which, though we have founded principally upon modern discoveries, is so remarkably coincident with one of a poetical and figurative character, drawn from tradition-, and handed down to us by the father of poetry, that we may he pardoned, perhaps, for introducing it here. According to Hesiod, Iris was the daughter of Thaumas (or Osiris) and Electra (or Isis), and trine sister of Aello and Ocypete. What are we to understand by this'; If Lit and her bow arc figurative, of colours, Electra of the active principle of light, and Thauman of the reactive principle of shade or darkness, as the learned will allow: and if Ocypete is also figurative of their offspring oxygen, and Attic [a stormy or gloomy air] also figurative of hydrogen, the reader will not find much difficulty in recon-ciling the poet's genealogy of Iris, or colours, with modem physics, and the chromatic theory we have founded upon upon-