270 THOMAS YOUNO. 



of Barton's buttons. A descendant of Mr. Barton has, 

 I believe, succeeded in reproducing the instrument 

 wherewith his grandfather obtained his brilliant effects. 

 But the greatest triumph of Young in this field was 

 the explanation of the beautiful phenomenon known as 

 Newton's rings. The colours of thin plates were pro- 

 fusely illustrated by the experiments of Hooke and 

 Boyle, but Newton longed for more than illustrations. 

 He desired quantitative measurement. The colour of 

 the film was known to depend upon its thickness. Can 

 this thickness be measured ? Here the unparalleled 

 penetration of Newton came into play. He took a lens, 

 consisting of a slice of a sphere of a diameter so large that 

 a portion of the curved surface of the lens approximated 

 to a plane surface. Upon this slightly convex surface he 

 placed a plate of glass the surface of which was accurately 

 plane. Squeezing them together, and allowing light 

 to fall upon them, he observed those beautiful iris- 

 circles with which his name will be for ever identified. 

 The iris-colours were obtained when he employed white 

 light. When monochromatic light was used he had 

 simply successive circles of light and darkness. Here 

 then, from the central point where the two glasses 

 touched each other, Newton obtained a film of air 

 which gradually increased in thickness as he retreated 

 from the point of contact. Whence this wonderful 

 recurrence of light and darkness ? The very constitu- 

 tion of light itself must be involved in the answer. 

 His desire was now to ascertain the thickness of the 

 film of air corresponding to the respective rings. 

 Knowing the curvature of his lens, this was a matter of 

 easy calculation. He measured the diameter of the 

 fifth ring of the series. This might be accurately done 

 with a pair of fine compasses, for the diameter was over 

 the fi fth of an inch in length. But it was the interval 



