irER^CHEL ON COLOURED RINGS. 121 



VII. 



Experiments Jbr investigating the Canse of the coloured con- 

 centric Rings, discovered by Sir Isaac Newton, between 

 two Object-Classes laid upon one another. By William 

 Herschel, £ZD. F.R.S*. 



XhE account given bv Sir I. Newton of the coloured Coloured rings 

 P Sir I. Newtou 



arcs and rings, which he discovered by laying two prisms or supposed, 



object-p-lasses upon each other, is highly interesting. He ma y Iea ? to * 



. . , , oj-mi completion of 



very justly remarks, that these phenomena are " or difficult the theory of 



consideration," but that " they may conduce to farther dis- ll S ht ' 



coveries for completing the theory of light, especially as to 



the constitution of the parts of natural bodies on which 



their colours or transparency dependf." 



With regard to the explanation of the appearance of His explana- 

 these coloured rings, which is given by Sir I. Newton, I unS itaheUKr* 

 must confess, that it has never been satisfactory to me. He 

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

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

 mission alternately returning and taking place with each ray 

 at certain stated intervals}. 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. i 



The great beauty of the coloured rings, and the pleasing Dr Herschel 

 appearances arising from the different degrees of pressure of ^ s^ect u> 

 the two surfaces of the glasses against each other when they some extent. 

 are formed, and especially the importance of the subject, 

 have often excited my desire of inquiring farther into the 

 cause of such interesting phenomena; 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, and the other of 170 faet focal length, 

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



• From the Phil. Tran?. for 1807, Part 111, p 180. 

 f Newton's Optics, 4th ed. p. 169. J Ibid. p. 256. 



many 



