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

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.

XV.

EXPLANATION OF THE FOREGOING PHENOMENA.

218.

Before we proceed further, it is incumbent on us to explain the first tolerably simple phenomenon, and to show its connexion with the principles first laid down, in order that the observer of nature may be enabled clearly to comprehend the more complicated appearances that follow.

219.

In the first place, it is necessary to remember that we have to do with circumscribed objects. In the act of seeing, generally, it is the circumscribed visible which chiefly invites our observation; and in the present instance, in speaking of the appearance of colour, as occasioned by refraction, the circumscribed visible, the detached object solely occupies our attention.

220.

For our chromatic exhibitions we can, however, divide objects generally into _primary_ and _secondary_. The expressions of themselves denote what we understand by them, but our meaning will be rendered still more plain by what follows.

221.

Primary objects may be considered firstly as _original_, as images which are impressed on the eye by things before it, and which assure us of their reality. To these the secondary images may be opposed as _derived_ images, which remain in the organ when the object itself is taken away; those apparent after-images, which have been circumstantially treated of in the doctrine of physiological colours.

222.

The primary images, again, may be considered as _direct_ images, which, like the original impressions, are conveyed immediately from the object to the eye. In contradistinction to these, the secondary images may be considered as _indirect_, being only conveyed to us, as it were, at second-hand from a reflecting surface. These are the mirrored, or catoptrical, images, which in certain cases can also become double images:

223.