3c8 NINETEENTH CENTURY. tt. hi. 



these colours arrange themselves in rings, beginning with a 

 dark spot at the top of the bubble and forming alternate 

 bands of blue, yellow, orange, and red, which grow fainter 

 and fainter down the sides of the bubble till they dis- 

 appear. The reason of these colours is that, when the 

 sunlight falls on the thin film of the bubble, a little of the 

 light is reflected straight back to the eye from the surface ^, 

 Fig. 50 ; but most of it passes on to the second surface b, 

 and there again some is reflected, so that two sets of waves 

 are constantly reaching the eye, one from a and one from b. 

 These two sets meet before they come to our eye, and we 

 have just seen (p. 306) that it depends entirely how they 

 meet whether we see light or darkness. 



Suppose the film is just thick enough for the two rays to 



Fig. sa 



Reflection of Light from the two surfaces of a Soap-bubble. 



R, Ray of light, part of which is reflected from the surface a, and part from the 



inner surface, b, to the eye. 



meet when the red waves of each are rising; we shall then 

 have a full red wave upon our eye. But in that case, as the 



* The film of a soap-bubble is really only the thickness of a fine 

 line even in the thickest part ; but it was necessary to exaggerate the 

 two surfaces in the diagram to show the passage of the ray of light. 



I 



