THE SIZE OF ATOMS. 167 



most interesting subjects of physical science to 

 admire. Blow a soap-bubble and look at it, 

 you may study all your life perhaps, and still 

 learn lessons in physical science from it. You 

 will now see on the screen the image of a soap- 

 film in a ring of metal. The light is reflected 

 from the film filling that ring, and made to focus 

 on the screen. It will show, as you see, colours 

 analogous to those of Newton's rings. As you 

 see it the image is upside down. The liquid 

 streams down (up in the image), and thins away 

 from the highest point of the film. First we see 

 that brilliant green colour. It will become 

 thinner and thinner there, and will pass through 

 beautiful gradations of colour till you see, as 

 now, a deep red, then much lighter, till it becomes 

 a dusky, yellowish-white, then green, and blue, 

 and deep violet, and lastly black, but after you 

 see the black spot it very soon bursts. The film 

 itself seems to begin to lose its tension when 

 its thickness gets considerably less than a quarter 

 of the wave-length of yellow light, which is 

 the thickness for the dusky white, preceding the 



