162 Prof. Airy's Remarks 



of the lights: he infers that the centre of the mixture of lights 

 ought to be the centre of the interference-fringes ; instead of 

 which it appears in experiment that on removing the eye and 

 eye-glass further and further from the prism, the interference- 

 fringes deviate from the centre of the mixed light more and 

 more towards the thick side of the prism, and finally disappear. 

 And this discordance is the point now to be explained. 



Suppose now that the light is not homogeneous (the reasons 

 for believing that this supposition is true will be given pre- 

 sently) ; and suppose only that the proportion of the differences 

 of the lengths of the waves, or if you please, of the lengths of 

 the fits, for all colours, from the length corresponding to some 

 particular colour, is the same as the proportion of the dif- 

 ferences of the deviation for those colours (produced by the 

 prism of the experiment) from the deviation corresponding to 

 that particular colour. This assumption is very nearly true 

 if the light comprehends rays from no more than one half or 

 one third of the spectrum. Let us now inquire what will be 

 the position of the fringes. 



Any one of the kinds of homogeneous light composing the 

 incident heterogeneous light will produce a series of bright and 

 dark bars, unlimited in number as far as the mixture of light 

 from the two pencils extends, and undistinguishable in quality. 

 The consideration, therefore, of homogeneous light will never 

 enable us to determine which is the point that the eye imme- 

 diately turns to as the centre of the fringes. What then is the 

 physical circumstance that determines the centre of the fringes? 



The answer is very easy. For different colours the bars have 

 different breadths. If then the bars of all colours coincide at 

 one part of the mixture of light, they will not coincide at any 

 other part; but at equal distances on both sides from that 

 place of coincidence they will be equally far from the state of 

 coincidence. If then we can find where the bars of all colours 

 coincide, that point is the centre of the fringes. 



It appears then that the centre of the fringes is not necessarily 

 the point where the two pencils of light have described equal 

 paths, but is determined by considerations of a perfectly dif- 

 ferent kind. And this is the radical error into which Mr: 

 Potter has fallen. The distinction is important in this and ill 

 other experiments. 



To find the centre of the fringes in Mr. Potter's experi- 

 ment, we must proceed thus: — First, we premise that the prism 

 produces greater deviation for yellow rays than for red rays, 

 and that the interference-bars are narrower for yellow rays 

 than for red rays. Next, we allow that the centre of the mix- 

 ture/??;- each colour is a point which the interfering pencils 



