270 THOMAS YOUNG. 



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 approxi- 

 mated to a 'plane surface. Upon this slightly convex sur- 

 face he placed a plate of glass the surface of which was 

 accurately plane. Squeezing them together, and allow- 

 ing light to fall upon them, he observed those beautiful 

 iris-circles with which his name will be for ever identi- 

 fied. 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 muse 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 fifth of an inch in length. But it was the interval 



