XIV.

CONDITIONS UNDER WHICH THE APPEARANCE OF COLOUR INCREASES.

209.

We have seen in the foregoing experiments that all appearance of colour occasioned by refraction depends on the condition that the boundary or edge be moved in upon the object itself, or the object itself over the ground, that the figure should be, as it were, carried over itself, or over the ground. And we shall now find that, by increased displacement of the object, the appearance of colour exhibits itself in a greater degree. This takes place in subjective experiments, to which, for the present, we confine ourselves, under the following conditions.

210.

First, if, in looking through parallel mediums, the eye is directed more obliquely.

Secondly, if the surfaces of the medium are no longer parallel, but form a more or less acute angle.

Thirdly, owing to the increased proportion of the medium, whether parallel mediums be increased in size, or whether the angle be increased, provided it does not attain a right angle.

Fourthly, owing to the distance of the eye armed with a refracting medium from the object to be displaced.

Fifthly, owing to a chemical property that may be communicated to the glass, and which may be afterwards increased in effect.

211.

The greatest change of place, short of considerable distortion of the object, is produced by means of prisms, and this is the reason why the appearance of colour can be exhibited most powerfully through glasses of this form. Yet we will not, in employing them, suffer ourselves to be dazzled by the splendid appearances they exhibit, but keep the above well-established, simple principles calmly in view.

212.

The colour which is outside, or foremost, in the apparent change of an object by refraction, is always the broader, and we will henceforth call this a _border_: the colour that remains next the outline is the narrower, and this we will call an _edge_.

213.

If we move a dark boundary towards a light surface, the yellow broader border is foremost, and the narrower yellow-red edge follows close to the outline. If we move a light boundary towards a dark surface, the broader violet border is foremost, and the narrower blue edge follows.

214.

If the object is large, its centre remains uncoloured. Its inner surface is then to be considered as unlimited (195): it is displaced, but not otherwise altered: but if the object is so narrow, that under the above conditions the yellow border can reach the blue edge, the space between the outlines will be entirely covered with colour. If we make this experiment with a white stripe on a black ground,[1] the two extremes will presently meet, and thus produce green. We shall then see the following series of colours:--

Yellow-red. Yellow. Green. Blue. Blue-red.

215.

If we place a black band, or stripe, on white paper,[2] the violet border will spread till it meets the yellow-red edge. In this case the intermediate black is effaced (as the intermediate white was in the last experiment), and in its stead a splendid pure red will appear.[3] The series of colours will now be as follows:--

Blue. Blue-red. Red. Yellow-red. Yellow.

216.

The yellow and blue, in the first case (214), can by degrees meet so fully, that the two colours blend entirely in green, and the order will then be,

Yellow-red. Green. Blue-red.

In the second case (215), under similar circumstances, we see only

Blue. Red. Yellow.

This appearance is best exhibited by refracting the bars of a window when they are relieved on a grey sky.[4]

217.

In all this we are never to forget that this appearance is not to be considered as a complete or final state, but always as a progressive, increasing, and, in many senses, controllable appearance. Thus we find that, by the negation of the above five conditions, it gradually decreases, and at last disappears altogether.

[1] Plate 2, fig. 5, _left_.

[2] Plate 2, fig. 5, _right_.

[3] This pure red, the union of orange and violet, is considered by the author the maximum of the coloured appearance: he has appropriated the term _purpur_ to it. See paragraph 703, and _note_.--T.

[4] The bands or stripes in fig. 4, plate 1, when viewed through a prism, exhibit the colours represented in plate 2, fig. 5.