CHEMICAL EFFECT IN DIOPTRICAL ACHROMATISM. 682. We first invite our readers to turn to what has been before observed on this subject (285, 298), to avoid unnecessary repetition here. 683. We can thus give a glass the property of producing much wider coloured edges without refracting more strongly than before, that is, without displacing the object much more perceptibly. 684. This property is communicated to the glass by means of metallic oxydes. Minium, melted and thoroughly united with a pure glass, produces this effect, and thus flint-glass (291) is prepared with oxyde of lead. Experiments of this kind have been carried farther, and the so-called butter of antimony, which, according to a new preparation, may be exhibited as a pure fluid, has been made use of in hollow lenses and prisms, producing a very strong appearance of colour with a very moderate refraction, and presenting the effect which we have called hyperchromatism in a very vivid manner. 685. In common glass, the alkaline nature obviously preponderates, since it is chiefly composed of sand and alkaline salts; hence a series of experiments, exhibiting the relation of perfectly alkaline fluids to perfect acids, might lead to useful results. 686. For, could the maximum and minimum be found, it would be a question whether a refracting medium could not be discovered, in which the increasing and diminishing appearance of colour, (an effect almost independent of refraction,) could not be done away with altogether, while the displacement of the object would be unaltered. 687. How desirable, therefore, it would be with regard to this last point, as well as for the elucidation of the whole of this third division of our work, and, indeed, for the elucidation of the doctrine of colours generally, that those who are occupied in chemical researches, with new views ever opening to them, should take this subject in hand, pursuing into more delicate combinations what we have only roughly hinted at, and prosecuting their inquiries with reference to science as a whole. PART IV. GENERAL CHARACTERISTICS. 688. We have hitherto, in a manner forcibly, kept phenomena asunder, which, partly from their nature, partly in accordance with our mental habits, have, as it were, constantly sought to be reunited. We have exhibited them in three divisions. We have considered colours, first, as transient, the result of an action and re-action in the eye itself; next, as passing effects of colourless, light-transmitting, transparent, or opaque mediums on light; especially on the luminous image; lastly, we arrived at the point where we could securely pronounce them as permanent, and actually inherent in bodies. 689.