edges will appear, they will spread in the same proportion, and when they meet, red will appear in the centre39. 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).
If we take away these dark objects from the prism, in which case, however, the glass is to be carefully cleaned, and hold a rod or a large pencil before the centre of the horizontal prism, we shall then accomplish the complete immixture of the violet border and the yellow-red edge, and see only the three colours, the external blue, and yellow, and the central red.
333.
If again we cut a long horizontal