50(J Erpermentsfor invest'} ^aibig, &c, 



tvith a puflT into the beam of light, while t kept my atten- 

 tion fixed tipon the Screen. As soon as the hair-powde^ 

 reached the beam of hght the screen was suddenly covered 

 with the most beautiful arrangement of concentric circles 

 displaying all the brilliant colours of the rainbow. A great 

 variety in the size of the rings was obtained by making the 

 Assistant strew the powder into the beam at a greater distance 

 from the mirror; for the rings contract by an increase of the 

 distance and dilate on a nearer approach of the powder. 



This cxpei-imcnt is so simple, and points out the genera! 

 Causes of the rmgs which are here produced in so plain a man^ 

 ner, that we may confidently say they arise from the flection of 

 (he rays of light on the particles of the floating powder, mo- 

 dified by the Curvature of the reflecting surface of the mirror. 



Here we have no interposed plate of glass of a given 

 thickness between one surface and another, that might pro- 

 duce the colours by reflecting some rays of light and trans- 

 mitting others; and if \Ve were inclined to look upon the 

 distance of the particles of the floating powder from the 

 mirror as plates of air, it would not be possible to assfgn 

 any certairi thickness to them, since these particles may be 

 Spread in the beam of light over a considerable space, and 

 perhaps none of them will be exactly at the same distance 

 from the mirror. 



I shall not enter into a further analysis of this experi* 

 ment, as the only purpose for which it is given in this place 

 is to show that the principle of thin or thick plates, either 

 of air or glass, on which the rays might alternately exert 

 their fits of easy reflection and easy transmission, must be 

 given up, and that the fits themselves of course cannot be 

 shown to have any exisistence. 



XXXIV. Conclusion. 

 It will hardly be necessary to say, that all the theory re- 

 lating to the size of the parts of natural bodies and their in- 

 terstices, which Sir I. Newton has founded upon the e:ti^- 

 tence of fits of easy reflection and easy transmission, exerted 

 differently, according to the different thickness of the thin 

 plates of which he supposes? the parts of natural bodies to 



consist. 



