SIR DAVID BREWSTER ON TPIE COLOURS OF THE SOAP-BUBBLE. 495 



this is effected, and the film placed in any position, the colour bands will be 

 again formed exactly as they would have been in a fresh film similarly placed. 

 The revolving rings may then be again produced, and the colouring matter again 

 combined with the soapy film. 



The colours which compose the two systems of rings may be exhibited by 

 holding the film vertically, when the colouring matter will arrange itself in bands 

 of the different orders to which it belongs. The bands thus exhibited are very 

 beautiful, and of great variety. 



The revolving systems of rings may be produced when the film is in any 

 other position. 



III. On the Form and Movements of the Bands and Rings in Convex and Concave Films. 



1. When convex and concave films are held vertically, the bands are formed 

 parallel to the horizon, exactly as in plane films, and the same phenomena take 

 place upon turning the glass round its axis. 



2. When a convex film is placed in a true horizontal position, and defended 

 from currents of air, the colours begin to form at the apex or summit of the film ; 

 first, a faint green or red of a high order, followed by coloured rings of inferior 

 orders, the first coloured rings descending till the film is covered with a regular 

 system, having black in its centre, and red and green rings occupying the circum- 

 ference of the film,* as in Fig. 11. 



This regular circular system may be produced more rapidly by first placing 

 the convex film vertically, and then, when the tints of several of the first orders 

 are developed, turning it cautiously into a horizontal position. The horizontal 

 bands will thus be converted, in a few seconds, into a regular concentric system. 

 This change is most easily effected with small and very convex films at the 

 mouth of closed tubes, about half-an-inch or three-quarters in diameter, in which 

 case we can increase the convexity even to a hemisphere by heating the tube 

 with the hand or otherwise. 



3. When a concave^ film is placed in a true horizontal position, and defended 

 from the air, the rings commence at the circumference of the film, and gradually 

 extend towards the centre, when they appear as in Fig. 12, in which there are 

 three orders of colours, from black of the first order to the red of the third order, 

 and within this is a lens having often a number of hardly visible rings within it. 

 This central portion of the colouring matter is obviously lenticular, as the di- 



* The order of these colours, as produced upon the upper hemisphere of the soap-buhble, is 

 described by Sir Isaac Newton in his Optics, p. 188. 



f It is difficult to obtain a good concave film by dipping the cylindrical wine-glass into the soap 

 solution. During the experiments of a whole day I never failed to obtain one, but with the same 

 glass and similar solutions I cannot now produce one. A certain mode of producing them will be 

 found in the following paper. 



VOL. XXIV. PART III. 6 S 



