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

In order to observe the phenomenon it is necessary to look obliquely on the surface where it appears. But, above all, it is to be remarked that by stooping still more, and looking at the appearance under a more acute angle, the circles not only grow larger but other circles are developed from the centre, of which no trace is to be discovered when we look perpendicularly, even through the strongest magnifiers.

443.

In order to exhibit the phenomenon in its greatest beauty, the utmost attention should be paid to the cleanness of the glasses. If the experiment is made with plate-glass adapted for mirrors, the glass should be handled with gloves. The inner surfaces, which must come in contact with the utmost nicety, may be most conveniently cleaned before the experiment, and the outer surfaces should be kept clean while the pressure is increased.

444.

From what has been said it will be seen that an exact contact of two smooth surfaces is necessary. Polished glasses are best adapted for the purpose. Plates of glass exhibit the most brilliant colours when they fit closely together, and for this reason the phenomenon will increase in beauty if exhibited under an air-pump, by exhausting the air.

445.

The appearance of the coloured rings may be produced in the greatest perfection by placing a convex and concave glass together which have been ground on similar segments of circles. I have never seen the effect more brilliant than with the object-glass of an achromatic telescope, in which the crown-glass and flint-glass were necessarily in the closest contact.

446.

A remarkable appearance takes place when dissimilar surfaces are pressed together; for example, a polished crystal and a plate of glass. The appearance does not at all exhibit itself in large flowing waves, as in the combination of glass with glass, but it is small and angular, and, as it were, disjointed: thus it appears that the surface of the polished crystal, which consists of infinitely small sections of lamellæ, does not come so uninterruptedly in contact with the glass as another glass-plate would.

447.

The appearance of colour vanishes on the strongest pressure, which so intimately unites the two surfaces that they appear to make but one substance. It is this which occasions the dark centre, because the pressed lens no longer reflects any light from this point, for the very same point, when seen against the light, is perfectly clear and transparent. On relaxing the pressure, the colours, in like manner, gradually diminish, and disappear entirely when the surfaces are separated.

448.

These same appearances occur in two similar cases. If entirely transparent masses become partially separated, the surfaces of their parts being still sufficiently in contact, we see the same circles and waves more or less. They may be produced in great beauty by plunging a hot mass of glass in water; the different fissures and cracks enabling us to observe the colours in various forms. Nature often exhibits the same phenomena in split rock crystals.

449.

This appearance, again, frequently displays itself in the mineral world in those kinds of stone which by nature have a tendency to exfoliate. These original lamellæ are, it is true, so intimately united, that stones of this kind appear altogether transparent and colourless, yet, the internal layers become separated, from various accidental causes, without altogether destroying the contact: thus the appearance, which is now familiar to us by the foregoing description, often occurs in nature, particularly in calcareous spars; the specularis, adularia, and other minerals of similar structure. Hence it shows an ignorance of the proximate causes of an appearance so often accidentally produced, to consider it so important in mineralogy, and to attach especial value to the specimens exhibiting it.

450.

We have yet to speak of the very remarkable inversion of this appearance, as related by men of science. If, namely, instead of looking at the colours by a reflected light, we examine them by a transmitted light, the opposite colours are said to appear, and in a mode corresponding with that which we have before described as physiological; the colours evoking each other. Instead of blue, we should thus see red-yellow; instead of red, green, &c., and _vice versâ_. We reserve experiments in detail, the rather as we have ourselves still some doubts on this point.

451.

If we were now called upon to give some general explanation of these epoptical colours, as they appear under the first condition, and to show their connexion with the previously detailed physical phenomena, we might proceed to do so as follows:--

452.