]g6 KERSCHEL ON COLOURED RINGS. 



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

 mitting others; and if we were inclined to look upon the 

 distance of the particles of the floating powder from the 

 minor as plates of air, it would not be possible to assign any 

 certain 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. 

 (1 I shall not enter into a further analysis of this experiment, 



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 existence. 



► XXXIV. Conclusion, 

 Newton's the- It will hardly fee necessary to say, that all the theory re- 



and°intersiice» ^ a ^ n S to tn ^ s * ze °^ tne P arts or * natural bodies and their in- 

 of bodies, terstices, which Sir I. Newton has founded upon the exist- 

 founded or i fits ence f £ ts f ^gy re fl ec ti n and easy trasmission, exerted 



of easy reflec- . , . 



tion aid trans- differently, according to the different thickness 01 the thm 

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

 ported by fact, consist, will remain unsupported; for if the above mentioned 

 fits have no existence, the whole foundation, on which the 

 theory of the size of such parts is placed, will be takeir away; 

 and we shaft consequently have to look out for a more firm 

 basis, on which a similar edifice may be placed. That there 

 is such a one we cannot doubt; and what I have already said 

 will lead us to look for it in the modifying power, which the 

 two surfaces, that have been proved to be essential to the 

 formation of rings, exert upon the rays of light. The Second 

 Part of this Paper, therefore, will enter into an examination 

 of tlie various modifications, that light receives in its approach 

 to, entrance into, or passage by, differently disposed surfaces 

 or bodies ; in order to discover, if possible, which of them 

 may be the immediate cause of the coloured rings that are 

 formed between glasses. 



VI. 



