THE SKY. 135 



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 * Newton's rings.' The 

 colours of his first circle, which immediately 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 re- 

 served for the illustrious Thomas Young, a man of in- 

 tellectual 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 

 inclosed between the plane and convex glasses. The 

 colours of thin plates were 'residual colours' survivals 

 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 

 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 particles 



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