the Cause of coloured concentric Rings, 73* 



ject-glasses upon each other, is highly interesting. He very 

 justly remarks, that these phsenomena are " of difficult con- 

 sideration,," but that *' they may conduce to further disco- 

 veries for completing the theory of light, especially as to 

 the constitution of the parts of natural bodies on which 

 their colours or transparency depend*.'* 



With regard to the explanation of the appearance of these 

 coloured rings, which is given by Sir I. Newton, I must 

 confess that it has never been satisfactory to me. He ac- 

 counts for the production of the rings, by ascribing to the 

 rays of light certain fits of easy reflection and easy trans- 

 mission alternat.ly returning and taking place with each ray 

 at certain stated intervals f. But this, without mentioning 

 particular objections, seems to be an hypothesis which can- 

 not be easily reconciled with the minuteness and extreme 

 velocity of the particles of which these rays^ according to 

 the Newtonian theory, are composed. 



The great beauty of the coloured rings, and the pleasing 

 appearances arising from the different degrees of pressure of 

 the two surfaces of the glasses against each other when they 

 are formed, and especially the importance of the subject, 

 have often excited my desire of inquiring further into the 

 cause of such interesting phsenomena ; and with a view to 

 examine them properly I obtained, in the year 1792, the 

 two object-glasses of Huygens, in the possession of the 

 Royal Society, one of 122 the other of 1 70 feet focal length, 

 and began a series of experiments with them, which, though 

 many times interrupted by astronomical pursuits, has often 

 been taken up again^ and has lately been carried to a very 

 considerable extent. The conclusions that may be drawn 

 from them, though they may not perfectly account for all 

 the phaenomena of the rings, are yet sufficiently well sup- 

 ported, and of such a nature as to point out several modifi- 

 cations of light that have been totally overlooked, and others 

 that have never been properly discriminated. It will, there- 

 fore, be the aim of this paper to arrange and distinguish the 

 various modifications of . light in a clear and perspicuous 



* Newton's Optics, i± ed.. p. 16D. f Ih'id. p. 956. 



order. 



