The question with us now is how to apply this contrivance to the working of a chromatrope. A chromatrope is a lantern slide consisting of two or more pieces so painted as to give the effect on the screen of a set piece of fireworks. It is the easiest of all slides to paint, as it is purely geometrical, and the colours used require no mixing, but can be put on at once as they are got from the shop. And, best of all, the effects produced are far and away more brilliant than anything else in lantern practice.
Fig. 3.
The essentials are a fixed slide of some bold design—star-shaped, circular, octagonal, or what not—and a moving shade that is of spiral pattern. The spiral pattern is the secret of the whole thing. Let the above be spun behind a star, so as to be only visible through its rays, and the star will seem to be alive. Turn it one way, the waves flow outwards; turn it the other, the waves flow inwards. The standing pattern must have a centre, and the spiral must be worked on the same centre. That done, all is well. Sometimes a double spiral is desirable, and then the pattern in [Fig. 3] is adopted. By it the outer part of the pattern (B) will seem to flow in a different direction from the inner (A). Paint the spirals any colours you please, providing that they are bright, and you get the results. Let reverse spirals appear in your fixed pattern, and you improve the effect. Have two revolving rings worked by the same wheel, one over the other, and the whole device will glow with life and brilliancy.
Fig. 4.
To begin modestly, take some simple design, such as that of the fountain in [Fig. 4]. Paint it on your slide, and stop it out all round with asphaltum or any dense black. Then fix to it a simple spiral disk, such as that given in [Fig. 2], and the instant the spiral spins the water will begin to flow.
In all designs remember that the fixed slide must be broken up with black and colour, so as not to show the whole of the spiral at once. The wave should, as it were, sweep over the windows. For front designs nothing is superior to a good knighthood star.
The colours for slide-painting can either be bought ready prepared, or ground down on plate glass and mixed by Canada balsam or mastic varnish. Should the balsam be too thick, use a little turpentine. The clearest colours are lampblack, burnt umber, burnt sienna, raw sienna, gamboge, Prussian blue, verdigris, and scarlet lake, and these are put on in the ordinary way. Before you begin clean the glass carefully with a little water and ammonia. Copy the design through the glass, and it would be as well not to forget that unless you can draw properly on paper you are not likely to do so on a lantern slide, where in the intense light and enlargement every mistake and shakiness of line is exaggerated for the benefit of the spectators. For those who cannot draw, figures of men and animals can be cut out with scissors and pasted on the slide, so as to give a shadow entertainment, as a change from the bought slides. Others can buy transparencies and stick them on for themselves, but the result will never equal hand-painting. One more caution before leaving hand-painting, and that is, do not be in a hurry, and give your slides at least a fortnight to dry.
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.