Optical projection: a treatise on the use of the lantern in exhibition and scientific demonstration (1906)

326 OPTICAL PROJECTION (b) For beautiful coloured bands, quickly developing: one volume oleate, J volume glycerine, and four volumes water. (c) For rapid development of the black spot: one volume oleate, five volumes water, and only a trace of glycerine. To use soap solutions we require a little apparatus, in the shape of wire rings. They are best made of iron, or tinned iron wire, with a stem bent at right angles from the junction, which must be soldered and then smoothed carefully off. Some about 2 inches diameter, as A (fig. 178), are used by nipping the stem horizontally (with the ring turned upwards) in a Bunsen holder as at c: some about 3 inches diameter are in- serted in small wooden feet as at B. Before use the rings should be made hot, and then rubbed with solid paraf- fin, which will run into a thin coating, and prevent the wire from cutting the films. Eings up to 4 or 5 inches diameter, dipped in a saucer of ' tough' solution and carefully lifted, will take up a film. Bubbles are best blown by a small glass funnel with ground edges, about an inch in diameter, to which is attached a small rubber tube for the mouth. With a tough solution I have blown a bubble over half a yard in diameter; and if a clean saucer be carefully soaped to the edge, and all froth avoided, a bubble nearly that size may be blown upon the saucer itself. A smaller size is however safer; and if the parallel beam from the lantern nozzle is deflected downwards upon this and reflected to the screen, fine colour-fringes will appear. If two or three stands, such as B in fig. 178, be FlG. 178.—Rings for Soap Films