566 Professor Tyndall [Jan. 22, 



duced. Young applied the theory successfully to explain the colours 

 of striated surfaces which, in the hands of Mr. Eutherfurd and others, 

 have been made to produce such splendid effects. The iridescences 

 on the polished surfaces of mother-of-pearl are due to the striae 

 produced by the edges of the shell-layers, which are of infinitesimal 

 thickness ; the fine lines drawn by Coventry, Wollaston, and Barton 

 upon glass also showed these colours. Barton afterwards succeeded 

 in transferring the lines to steel and brass. Most of you are ac- 

 quainted with the iridescence of Barton's buttons. A descendant of 

 Mr. Barton has, I believe, succeeded in reproducing the instrument 

 wherewith his grandfather produced his brilliant effects. 



But the greatest triumph of Young in this field was the explana- 

 tion of the beautiful phenomenon known as Newton's rings. The 

 colours of thin plates were profusely 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 unparalled penetration of Newton came into play. He took a 

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

 curved surface of the lens approximated to a plane surface. Upon 

 this slightly convex surface he placed a plate of glass whose surface 

 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 constitution 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 fifth of an inch in length. 

 But it was the interval between the glasses corresponding to this 

 distance that Newton required to know, and this he found by calcula- 

 tion to be 1-37, 000th of an inch. This, be it remembered, is the 

 distance corresponding to the fifth ring. The interval corresponding 

 to the first ring would be only a fifth of this, or, in other words, about 

 l-180,000th of an inch. Such are the magnitudes with which we have 

 to deal before the question " What is Light ? " can be scientifically 

 answered. 



Newton's explanation of the rings, which he was the first to 

 discover, though artificial in the highest degree, is marked by his 

 profound sagacity. He was hampered by the notion of the " cor- 

 poreity" of light. He could not get over the objection raised by 



