STRETCHED FILMS OF LIQUID. 



PROFESSOR C. V. BOYS' RAINBOW CUP. 

 By E. S. GREW, M.A. 



One of the chief interests of the liquid film, as displayed, for 



example, in a soap bubble, is that its tenuity is such as almost 



to bring the observer within reach of molecules. An interest 



which is even more popular, and which is not on that account 



of less scientific usefulness, is 



the extreme beauty of the 



reflections of light from the 



double surface of a film. 



These are apparent in that 



commonest of objects, the 



soap bubble, but unless one 



has the manipulative dexterity 



of Mr. C. V. Boys a soap 



bubble is difficult to examine 



in detail or at leisure. At the 



anniversary meeting of the 



Royal Society, however, Mr. 



Boys exhibited a device which 



allows the least accomplished 



observer to vivisect a soap 



bubble, and to lay bare the 



changes which, owing to the 



progressive thinning of the 



film, it undergoes in its life 



history. 



The device he has since 

 called the " Rainbow Cup," a 

 title which is readily explained 

 by the most characteristic 

 appearance of the film when 

 stretched across the crater of 

 Mr. Boys' Cup. The cup is a 

 revolving brass drum, across 

 the top of which it is easy to 

 stretch a film or membrane of 

 soap solution. When the drum 

 is set spinning the film thins 

 from its centre to its circum- 

 ference, partly as the result of 

 centrifugal action ; partly, 

 because where the upper and 

 lower surfaces of the film join 

 the edge of the drum a sort of 

 suction pump action is con- 

 tinually going on. A film, which 

 is practically a sandwich of 

 liquid between two stretched 

 skins, is always joined to a solid 

 surface like the edge of the 

 drum in a conical formation, 

 roughly, thus: — 



> <, 



and the V's at either end of the film are always sucking away 

 the liquid in the film. 



Now as everyone knows there is a constant play of 

 iridescent colour on the surface of a soap-bubble. This is 

 due to the rays of light reflected from it being reflected from the 

 inner skin, as well as from the outer skin of the film, just as in 

 a looking glass one may sometimes see two reflections, one 

 image arising from the glass surface, the other from the 

 silvered back. Consider now the case of red rays of a fixed 

 wave length of light falling on the two surfaces of the soap 

 bubble's film. If the two surfaces are exactly the proper 

 distance apart, the waves coming back from the inner reflect- 

 ing surface, may exactly meet and neutralise the waves 



Figure 115. 

 Professor C. V. Boys' Rainbow Cup 



reflected from the outer surface — the crests of one series of 

 waves coming exactly where the troughs of the other series 

 of waves are situated. If, however, the twin surfaces are 

 " half a wave length of red light " further apart, or half a 



wave length of red light nearer 

 together, the crests and troughs 

 of the two sets of waves will 

 not neutralise, but will rein- 

 force one another. Thus we 

 can easily imagine a red reflec- 

 tion from the film appearing 

 and disappearing and appear- 

 ing again, as the film thinned to 

 appropriate dimensions. But 

 the different colours of light 

 have different wave lengths; so 

 that when the film fails to 

 reflect one kind of light, it may 

 yet be reflecting another. The 

 thickness of the film unsuitable 

 for showing up the red or the 

 pink may be suitable for 

 reflecting the green : then as 

 the film grows thinner the 

 pink's turn comes again and 

 the green disappears, and so 

 into the other colours, the 

 blues, violets, straw colours, 

 into which during the whole of 

 its life the film is analysing the 

 white or yellow light falling 

 on it. 



These effects are apprehend- 

 ed, but are all mixed together 

 on an ordinary soap bubble 

 film : but in the film of Mr. 

 Boys' revolving cup, the 

 regularly diminishing thick- 

 ness gives them an orderly 

 succession. As the film be- 

 comes thinner the colours 

 appear in circular rainbow 

 bands in the following order, 

 beginning with a film fifty 

 millimetres of an inch thick : — 

 pale green, pink, pale green, 

 pink, bluish green, salmon 

 colour, bright green, magenta, 

 yellowish and then brilliant 

 apple green, blue, purple, red, 

 yellow, poor white, steel blue, 

 purple, brown, straw colour, 

 white, black. Each of these 

 colours is related to its particular thickness of film which can 

 be computed. The apple green is twenty millimetres of an 

 inch thick. When the black is reached, far below the wave 

 length of any light, the thickness of the film is only stfirciTTrath 

 of an inch thick. 



The black appears in a small spot at the centre of the cup 

 (and of the spinning rainbow bands) and slowly grows. It 

 can be made to grow faster by stopping the rotation and 

 tilting the drum. Then the black film being thinnest and, 

 therefore, lightest, passes upwards, to the higher edge of the 

 drum. It continually eats its way into the other colours of 

 the film and Professor Boys in his paper in the Royal Society's 

 Proceedings surmises that the areas of black thus formed, 



115 



