THE SIZE OF A TOMS. 169 



Part I.) tells more of this important phenomenon 

 of the black spot than is known to many of 

 the best of modern observers. 



"Obs. 17. If a bubble be blown with water, 

 first made tenacious by dissolving a little soap 

 in it, it is a common observation that after a 

 while it will appear tinged with a variety of colours. 

 To defend these bubbles from being agitated 

 by the external air (whereby their colours are 

 irregularly moved one among another so that 

 no accurate observation can be made of them), 

 as soon as I had blown any of them I covered 

 it with a clear glass, and by that means its 

 colours emerged in a very regular order, like 

 so many concentric rings encompassing the top 

 of the bubble. And as the bubble grew thinner 

 by the continual subsiding of the water, these 

 rings dilated slowly and overspread the whole 

 bubble, descending in order to the bottom of it, 

 where they vanished successively. In the mean- 

 while, after all the colours were emerged at the 

 top, there grew in the centre of the rings a 

 small round black spot like that in the first 



