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).