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,
- By a more oblique direction of the impinging luminous object through mediums with parallel surfaces.
- By changing the parallel form for one more or less acute angled.
- By increased proportion of the