144 FRAGMENTS OF SCIENCE 



thickness of tlie film and the color it produced, varied 

 Hooke's experiment. Taking two pieces of glass, the one 

 plane and the other very slightly curved, and pressing 

 both together, he obtained a film of air of gradually in- 

 creasing thickness from the place of contact outward. 

 As he expected, he found the place of contact surrounded 

 by a series of colored circles, still known all over the 

 world as *'!N"ewton's rings." The colors of his first cir- 

 cle, which immediately surrounded a black central spot, 

 Newton called "colors of the first order"; the colors of 

 the second circle, "colors of the second order," and so 

 on. With unrivalled penetration and apparent success, 

 he applied his theory of "fits" to the explanation of the 

 "rings." Here, however, the only immortal parts of his 

 labors are his facts and measurements; his theory has 

 disappeared. It was reserved for the illustrious Thomas 

 Young, a man of intellectual calibre resembling that of 

 Newton himself, to prove that the rings were produced 

 by the mutual action — in technical phrase, "interference" 

 — of the light- waves reflected at the two surfaces of the 

 film of air enclosed between the plane and convex glasses. 

 The colors of thin plates were "residual colors"- — surviv* 

 als of the white light after the ravages of interference. 

 Young soon translated the theory of "fits" into that of 

 "waves"; the measurements pertaining to the former be- 

 ing so accurate as to render them immediately available 

 for the purposes of the latter. 



It is here that Newton's researches and opinions touch 

 the subject of this article. The color nearest to the black 

 spot, in the experiment above described, was a faint blue 

 — "blue of the first order" — corresponding to the film of 

 air when thinnest. If a solid or liquid film, of the thick- 



