If the spaces of the chromatrope are too narrow, or the design is worked too fast, you will only produce a fog, just as you do when you spin a toothed wheel. If, however, the teeth of the two wheels be fixed on the slant and the wheels turned in opposite directions, the teeth will be visible--upright if the teeth slant the way the wheel turns, slanting if they follow the course of the wheel. The principle holds good with the chromatrope; the spiral is but a toothed wheel.
The chromatrope was invented by Mr. W. Allen. He called it the pyreidotrope. It was improved, and soon became known as Chinese fireworks. From it the paper fireworks still popular in some circles were derived.
Paper fireworks are merely large chromatropes painted on oiled paper instead of on glass, and lighted by candles instead of a lantern. The wheels, and stars, and plumes, and pyramids are just the same as we have been describing, only much larger. The transparent portions are got by perforations. The machinery is the same, but of a rougher pattern, and the moving parts, instead of reels and bobbins, are barrel-hoops and butter-tubs. From what has been said above, their construction is sufficiently obvious, and we conclude with a design (Fig. 5) that may serve for either purpose.
Have our readers ever tried to work these together? If not, let them do so, and they will be rather surprised at the results. Begin in a small way, and treat the screen as the broad end of the kaleidoscope. Arrange it at an angle to the audience. Against it place two mirrors at the desired angle, just as in the well-known instrument, and then damp the screen and throw the image on from behind, so that it will take the place of the usual coloured fragments.
Chromatropes can be made in this way by devising a double slide, and filling up the space between the two sides with stained splinters of glass, beads, etc.; and this contrivance put into the lantern, and its image thrown on to the screen between the mirrors, will be multiplied into the most brilliant geometrical fireworks, whose pattern can be varied as often as desired by simply shaking the glass slide, so as to rearrange the coloured odds and ends. If the slide is a mechanical chromatrope, of course so much the better, as instead of one image of it there are many. Modifications of this experiment will easily suggest themselves to an ingenious lad.