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XXXV. On some Properties of Soap-bubbles. 

 By J. Broughton, B.Sc, Chemical Assistant, Royal Institution* . 



THE colours of a soap-bubble, proverbial for their beauty, 

 have long furnished an apt illustration of the optical 

 effects produced by thin films; but the fragility of the subject 

 has probably prevented their closer examination by other means 

 than that of the unaided eye. It is, however, well known that 

 those parts of the bubble which at a short distance appear of one 

 homogeneous tint reveal, on closer inspection, remarkable streaks 

 and curves of various brilliant colours, which doubtless denote 

 portions of the bubble where equal thicknesses of film produce 

 the interference of the same set of rays. In a thin bubble it is 

 rarely that a space exists of a millimetre square without many 

 of these streaks being visible, and it is only by the preponder- 

 ance of one particular tint that the effect of a uniformly coloured 

 surface of considerable extent is produced. The oleate-of-soda 

 and glycerine solution, invented by M. Plateau, renders the em- 

 ployment of soap-bubbles for many experimental purposes now 

 comparatively easy ; and the vexatious bursting of the bubble at 

 the moment of observation is nearly entirely obviated by their 

 remarkable persistency. Bubbles blown with this solution are 

 admirably adapted for optical experiments. 



A bubble thus formed and placed on a wire ring under a glass 

 shade will, after standing for an hour, frequently exhibit at its 

 upper pole a circular black spot one-third or even half an inch 

 in diameter. The black is intense, but it always possesses the 

 property of reflecting a small amount of light. In this position 

 it is easy to examine it by means of a lens, which renders evident 

 optical effects of great splendour and interest, and moreover 

 reveals the film to be always in motion. These appearances 

 suggested the employment of a compound microscope for their 

 observation. For this purpose a strip of cork of a convenient 

 size for the stage of the microscope was covered with black 

 paper, and on this a small wire ring was placed, on which a bub- 

 ble was blown by means of a caoutchouc tube of small bore. The 

 bubble was strongly illuminated by a good condenser, so that 

 the light after reflexion might pass through the microscope. 



When thus viewed, the film exhibits optical phenomena of the 

 utmost magnificence, which are difficult to describe. The ap- 

 pearances observed in and near the black spaces above referred to, 

 were of especial splendour. On the black ground moved specks 

 of brilliant yellow and orange, which again contained smaller 

 spots of blue and black of almost every geometrical form, but 



* Cominimicatert by the Author. 



