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 



