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XXXIII — On the Colours of the Soap-Bubble. By Sir David Brewster, K.H., F.R.S. 



(Plate XXXIII.) 



(Read 21st January 1867.) 



The phenomena and colours of the soap-bubble have been the subject of 

 more frequent observation than any other facts in science. For nearly two 

 centuries they have afforded amusement and instruction to the young, and 

 have exercised the genius of the most distinguished philosophers. Hook* and 

 Newton f ascribed the beautiful colours of the soap-bubble to the different 

 degrees of thickness of the attenuated film, and this opinion has been im- 

 plicitly adopted by every optical writer down to the present day. In the latest 

 and best Treatise on Light, Sir John Herschel % expresses the current theory 

 more distinctly than others, when he says, " that the brilliant colours which 

 appear on soap-bubbles consist of a regular succession of hues disposed in 

 the same order, and determined, obviously not by any colour in the medium 

 itself, in which they are formed, or on whose surfaces they appear, but solely by 



its greater or less thickness It is at first uniformly white, but as it 



grows thinner and thinner, by the subsidence of its particles, colours appear to 

 begin at its top, where thinnest, which grow more and more vivid, and arrange 

 themselves in beautiful horizontal zones about the highest point, as a centre." 



Of the correctness of this theory I never entertained a doubt, till I resumed 

 the study of the subject, while repeating the beautiful experiments of Professor 

 Plateau, " On the Figures of equilibrium of a liquid mass without gravity." 

 The colours exhibited by plane, convex, and concave liquid films thus came under 

 my notice, and led me to the true cause of the colours of the soap-bubble. 



Dr Thomas Young § is the only author, so far as I know, that has described 

 these colours, as produced by a film stretched across the mouth of a wine-glass, 

 and it is strange that he did not observe certain changes in the coloured bands 

 produced by the simple motion of the glass, which might have led him to their 

 true explanation. He contents himself with giving an incorrect coloured draw- 

 ing of the bands in their first stage, and adopts the usual theory of their forma- 

 tion. [| 



* Birch's Hist, of the Royal Society, vol. iii. p. 29. 



\ Optics, 3d edition, p. 187. 



J Treatise on Light, § 633, p. 462. 



§ Elements of Natural Philosophy, vol. i. p. 469, and plate xxx. fig. 448. The dark band in 

 this figure does not exist. 



|| " When a film of soapy water," says Dr Young, " is stretched over a wine-glass, and placed 

 in a vertical position, its upper edge becomes extremely thin, and apparently black, while the parts 

 below are divided by horizontal lines into a series of coloured bands." — Id. p. 458. 



VOL. XXIV. PART III. 6 R 



