Theory of Colours — trans. Charles Lock Eastlake (1840)

To ascertain the effect of coloured light on oxydation and de-oxydation, the following means may be employed:--Let moist, perfectly white muriate of silver[1] be spread on a strip of paper; place it in the light, so that it may become to a certain degree grey, and then cut it in three portions. Of these, one may be preserved in a book, as a specimen of this state; let another be placed under a yellow-red, and the third under a blue-red glass. The last will become a darker grey, and exhibit a de-oxydation; the other, under the yellow-red glass, will, on the contrary, become a lighter grey, and thus approach nearer to the original state of more perfect oxydation. The change in both may be ascertained by a comparison with the unaltered specimen.

681.

An excellent apparatus has been contrived to perform these experiments with the prismatic image. The results are analogous to those already mentioned, and we shall hereafter give the particulars, making use of the labours of an accurate observer, who has been for some time carefully prosecuting these experiments.[2]

[1] Now generally called chloride of silver: the term in the original is Hornsilber.--T.

[2] The individual alluded to was Seebeck: the result of his experiments was published in the second volume.--T.

LVI.

CHEMICAL EFFECT IN DIOPTRICAL ACHROMATISM.

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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.

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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.

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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.

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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.

In following this order we have as far as possible endeavoured to define, to separate, and to class the appearances. But now that we need no longer be apprehensive of mixing or confounding them, we may proceed, first, to state the general nature of these appearances considered abstractedly, as an independent circle of facts, and, in the next place, to show how this particular circle is connected with other classes of analogous phenomena in nature.

THE FACILITY WITH WHICH COLOUR APPEARS.

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We have observed that colour under many conditions appears very easily. The susceptibility of the eye with regard to light, the constant re-action of the retina against it, produce instantaneously a slight iridescence. Every subdued light may be considered as coloured, nay, we ought to call any light coloured, inasmuch as it is seen. Colourless light, colourless surfaces, are, in some sort, abstract ideas; in actual experience we can hardly be said to be aware of them.--Note Z.

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