•ect. in.] DISSERTATION SF.COND. 77 



minute parts, the mere particles of all bodies, even the most 

 opaque, are transparent, they may very well be conceived to 

 act on light after the manner of the thin plates, and to pro- 

 duce each, according to its thickness and density, its appro- 

 priate colour, which, therefore, becomes the colour of the 

 surface. Thus the colours in which the bodies round us ap- 

 pear everywhere arrayed, are reducible to the action of the 

 parts, which constitute their surfaces, on the refined and active 

 fluid which pervades, adorns, and enlightens the world. 



But the same experiments led to some new and unexpect- 

 ed conclusions, that seemed to reach the very essence of the 

 fluid of which we now speak. It was impossible to observe, 

 without wonder, the rings alternately luminous and dark that 

 were formed between the two plates of glass in the preced- 

 ing experiments, and determined to \>e what they were by 

 the different thickness of the air between the plates, and 

 having to that thickness the relations formerly expressed. 

 A plate of which the thickness was equal to a certain quan- 

 tity multiplied by an odd number, gave always a circle of the 

 one kind ; but if the thickness of the plate was equal to the 

 same quantity multiplied by an even number, the circle was 

 of another kind, the light, in the first case, being reflected, 

 ' in the second transmitted. Light penetrating a thin transpa- 

 rent plate, of which the thickness was m, 3m, 5m, &c. was 

 decomposed and reflected ; the same light penetrating the 

 same plate, but of the thickness 0, 2m, 4m, was transmitted, 

 though, in a certain degree, also decomposed. The same 

 light, therefore, was transmitted or reflected, according as 

 the second surface of the plate of air through which it passed 

 was distant from the first, by the intervals 0, 2, 4m, or m, 3m, 

 5m ; so that it becomes necessary to suppose the same ray 

 to be successively disposed to be transmitted and to be re- 

 flected at points of space separated from one another by the 

 same interval m. This constitutes what Newton called Fits 



