106 Mayleigh's Address to the British Association. 



the velocity of light in vacuo varies from color to color, to such 

 an extent that the velocity of blue light is nearly two per cent, 

 greater than that of red light. Such a variation is quite opposed 

 to existing theoretical notions, and could only be accepted on the 

 strongest evidence. Mr. Michelson, whose method (that of Fou- 

 cault) is well suited to bring into prominence a variation of 

 velocity with wave-length, informs me that he has recently repeated 

 his experiments, with special reference to the point in question, 

 and has arrived at the conclusion that no variation exists compar- 

 able with that asserted by Young and Forbes. The actual 

 velocity differs little from that found from his first series of experi- 

 ments, and may be taken to be 299,800 kilometres per second. 



It is remarkable how many of the playthings of our childhood 

 give rise to questions of the deepest scientific interest. The top 

 is or may be understood, but a complete comprehension of the 

 kite and of the soap-bubble would carry us far beyond our present 

 stage of knowledge. In spite of the admirable investigations of 

 Plateau, it still remains a mystery why soapy water stands almost 

 alone among fluids as a material for bubbles. The beautiful 

 development of color was long ago ascribed to the interference of 

 light, called into play by the gradual thinning of the film. In 

 accordance with this view the tint is determined solely by the 

 thickness of the film and the refractive index of the fluid. Some 

 of the phenomena are however so curious as to have led excellent 

 observers, like Brewster, to reject the theory of thin plates, and to 

 assume the secretion of various kinds of coloring matter. If the 

 rim of a wine-glass be dipped in soapy water, and then held in a 

 vertical position, horizontal bands soon begin to show at the top 

 of the film, and extend themselves gradually, downwards. Accord- 

 ing to Brewster these bands are not formed by the " subsidence 

 and gradual thinning of the film," because they maintain their 

 horizontal position when the glass is turned round its axis. The 

 experiment is both easy and interesting ; but the conclusion 

 drawn from it cannot be accepted. The fact is that the various 

 parts of the film cannot quickly alter their thickness, and hence 

 when the glass is rotated they re-arrange themselves in order of 

 superficial density, the thinner parts floating up over, or through 

 the thicker parts. Only thus can the tendency be satisfied 

 the centre of gravity to assume the lowest possible position. 



