200 Lord Rayleigh on Achromatic 



unaltered, the width is proportional to t~\ Hence, as the 

 interval between the surfaces increases, the bands become 

 finer, but the centre does not shift, nor is there any change in 

 their number as limited by the advent of chromatic confusion. 



Effect of a Prism wpon Newton's Rings. 



If Newton's rings are examined through a prism, some very 

 remarkable phenomena are exhibited, described in his 24th 

 observation*. 



" When the two object-glasses were laid upon one another, 

 so as to make the rings of the colours appear, though with my 

 naked eye I could not discern above 8 or 9 of these rings, yet 

 by viewing them through a prism I have seen a far greater 

 multitude, insomuch that I could number more than 40, besides 

 many others which were so very small and close together that 

 I could not keep my eye steady on them severally so as to 

 number them, but by their extent I have sometimes estimated 

 them to be more than a hundred. And I believe the experi- 

 ment may be improved to the discovery of far greater 

 numbers ; for they seem to be really unlimited, though visible 

 only so far as they can be separated by the refraction, as 

 I shall hereafter explain. 



" But it was but one side of these rings — namely, that 

 towards which the refraction was made — which by that 

 refraction was rendered distinct ; and the other side became 

 more confused than when viewed by the naked eye, insomuch 

 that there I could not discern above 1 or 2, and sometimes 

 none of those rings, of which I could discern 8 or 9 with my 

 naked eye. And their segments or arcs, which on the other 

 side appeared so numerous, for the most 

 part exceeded not the third part of a ^ 3 - 



circle. If the refraction was very great, 

 or the prism very distant from the 

 object-glasses, the middle part of those 

 arcs became also confused, so as to dis- 

 appear and constitute an even white- 

 ness, while on either side their ends, 

 as also the whole arcs furthest from the 

 centre, became distincter than before, 

 appearing in the form as you see them 

 designed in the fifth figure [fig. 3]." 



" The arcs, where they seemed distinctest, were only black 

 and white successively, without any other colours intermixed. 

 But in other places there appeared colours, whose order was 



* Opticks. See also Place, Pogg. Ann. cxiv. p. 504 (1861). 



