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

The observer may convince himself in like manner of the mode in which the appearance of colour takes place in the diagonal direction when the displacement is effected by means of two prisms, as has been plainly enough shown in the subjective example; for this experiment, however, prisms should be procured of few degrees, say about fifteen.

321(206, 207).

That the colouring of the image takes place here too, according to the direction in which it moves, will be apparent if we make a _square_ opening of moderate size in a shutter, and cause the luminous image to pass through a water-prism; the spectrum being moved first in the horizontal and vertical directions, then diagonally, the coloured edges will change their position accordingly.

322(208).

Whence it is again evident that to produce colour the boundaries must be carried over each other, not merely move side by side.

XXIII.

CONDITIONS OF THE INCREASE OF COLOUR.

323 (209).

Here too an increased displacement of the object produces a greater appearance of colour.

324 (210).

This increased displacement occurs,

1. By a more oblique direction of the impinging luminous object through mediums with parallel surfaces.

2. By changing the parallel form for one more or less acute angled.

3. By increased proportion of the medium, whether parallel or acute angled; partly because the object is by this means more powerfully displaced, partly because an effect depending on the mere mass co-operates.

4. By the distance of the recipient surface from the refracting medium so that the coloured spectrum emerging from the prism may be said to have a longer way to travel.

5. When a chemical property produces its effects under all these circumstances: this we have already entered into more fully under the head of achromatism and hyperchromatism.

325 (211).

The objective experiments have this advantage that the progressive states of the phenomenon may be arrested and clearly represented by diagrams, which is not the case with the subjective experiments.

326.

We can observe the luminous image after it has emerged from the prism, step by step, and mark its increasing colour by receiving it on a plane at different distances, thus exhibiting before our eyes various sections of this cone, with an elliptical base: again, the phenomenon may at once be rendered beautifully visible throughout its whole course in the following manner:--Let a cloud of fine white dust be excited along the line in which the image passes through the dark space; the cloud is best produced by fine, perfectly dry, hair-powder. The more or less coloured appearance will now be painted on the white atoms, and presented in its whole length and breadth to the eye of the spectator.

327.

By this means we have prepared some diagrams, which will be found among the plates. In these the appearance is exhibited from its first origin, and by these the spectator can clearly comprehend why the luminous image is so much more powerfully coloured through prisms than through parallel mediums.

328 (212).

At the two opposite outlines of the image an opposite appearance presents itself, beginning from an acute angle;[1] the appearance spreads as it proceeds further in space, according to this angle. On one side, in the direction in which the luminous image is moved, a violet border advances on the dark, a narrower blue edge remains next the outline of the image. On the opposite side a yellow border advances into the light of the image itself, and a yellow-red edge remains at the outline.

329 (213).

Here, therefore, the movement of the dark against the light, of the light against the dark, may be clearly observed.

[Illustration]

330 (214).

The centre of a large object remains long uncoloured, especially with mediums of less density and smaller angles; but at last the opposite borders and edges touch each other, upon which a green appears in the centre of the luminous image.

331 (215).

Objective experiments have been usually made with the sun's image: an objective experiment with a dark object has hitherto scarcely been thought of. We have, however, prepared a convenient contrivance for this also. Let the large water-prism before alluded to be placed in the sun, and let a round pasteboard disk be fastened either inside or outside. The coloured appearance will again take place at the outline, beginning according to the usual law; the edges will appear, they will spread in the same proportion, and when they meet, red will appear in the centre[2]. An intercepting square may be added near the round disk, and placed in any direction _ad libitum_, and the spectator can again convince himself of what has been before so often described.

332 (216).