ON LIGHT. 287 



tion of the " law of interference " of the rays of light : a 

 law we shall now proceed to explain, taking for our 

 first exemplification of it this very phenomenon. 



(70.) If a soap-bubble be blown in a clean circular 

 saucer with a very smooth, even rim, well moistened 

 with the soapy liquid,* and care be taken in the blowing 

 that it be single, quite free from any small adhering 

 bubbles, and somewhat more than hemispherical ; so 

 that, while it touches and springs from the rim all round, 

 it shall somewhat overhang the saucer : and if in this 

 state it be placed under a clear glass hemisphere or 

 other transparent cover to defend it from gusts of air 

 and prevent its drying too quickly ; the colours, which 

 in the act of blowing wander irregularly over its surface, 

 will be observed to arrange themselves into regular 

 circles surrounding the highest point or vertex of the 

 sphere. If the bubble be a thick one (z>., not blown to 

 near the bursting point), only faint, or perhaps no colours 

 at all will at first appear, but will gradually come on 

 growing more full and vivid, and that, not by any par- 



* M. Plateau gives the following recipe for such a liquid, i. 

 Dissolve one part, by weight, of Marseilles soap, cut into thin slices 

 in forty parts of distilled water, and filter. Call the filtered liquid 

 A. 2. Mix two parts, by measure, of pure glycerine with one part 

 of the solution A, in a temperature of 66 Fahr., and after shaking 

 them together long and violently, leave them at rest for some days. 

 A clear liquid will settle, with a turbid one above. The lower is 

 to be sucked out from beneath the upper with a siphon, taking the 

 utmost care not to carry down any of the latter to mix with the 

 clear fluid. A bubble blown with this will last several hours even 

 in the open air. Or, the mixed liquid, after standing twenty-four 

 hours, may be filtered. 



