THE SKY. 135 



tained a film of air of gradually increasing thickness 

 from the place of contact outwards. As he expected, 

 he found the place of contact surrounded by a series of 

 coloured circles, still known all over the world as ' New- 

 ton's rings.' The colours of his first circle, which im- 

 mediately surrounded a black central spot, Newton 

 called ' colours of the first order; ' the colours of the 

 second circle, ' colours 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 

 labours 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, ' interfer- 

 ence ' of the light-waves reflected at the two surfaces 

 of the film of air inclosed between the plane and convex 

 glasses. The colours of thin plates were * residual col- 

 ours ' survivals of the white light after the ravages of 

 interference. Young soon translated the theory of 

 ' fits ' into that of ' waves; ' the measurements pertain- 

 ing to the former being 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 colour 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 thickness requisite to produce this colour, 

 were broken into bits and scattered in the air, Newton 

 inferred that the tiny fragments would display the blue 

 colour. Tantamount to this, he considered, was the ac- 

 tion of minute water-particles in the incipient stage of 

 their condensation from aqueous vapour. Such parti- 

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