224 



HISTORY OF SCIENCE. 



ing the phenomena in this way, Newton obtained the precise thickness 

 of the air-film which corresponded with each simple colour. Fig. 109 

 represents the appearance of the light and dark rings on a large scale, 

 when the lenticular air-film is viewed by reflected light. When light is 

 allowed to pass through, another system of rings is perceived, exactly 



FIG. 109. 



the reverse of the former, i.e., the centre is now a light spot, and the 

 space which the reflected light presented as a luminous ring now ap- 

 pears dark. Thus it appeared that, at those thicknesses where the light 

 was not reflected, it was transmitted. A red ray, for example, was re- 

 flected or transmitted according as it fell upon one or another thick- 

 ness of the air-film, differing from another part by an extremely small 

 but assignable amount. Thus Newton found that at the places where 

 certain bright rings were formed, the thicknesses were successively the 

 following zySjOooth parts of an inch 



13579 etc. ; 

 while the thicknesses corresponding with the dark rings were, reckon- 

 ing in the same fraction of an inch, 



2 4 6 8 10 etc. 



Newton expressed these facts by a supposition that along a ray of light 

 there were regularly alternating positions where the light was disposed 

 to be in turns readily reflected and readily transmitted, or, as he ex- 

 pressed it, the ray had alternate^/.? of easy reflection and of easy transmis- 

 sion. Although Newton inclined to that theory of light which regards 

 it as consisting of a material emanation and the mode in which he has 



