164 Prof. Airy's Remarks 



of the prism. If 8 d and I b have the same proportion for rays of 

 different colours (which we have mentioned above to be very 

 nearly accurate), then in a given position of the eye-glass the 

 union of colours will be perfect at the place which we have 

 found for the centre of the fringes; and the same place would 

 be found for the centre of the fringes if a part of the colours 

 were abstracted, or if the light were made more nearly ho- 

 mogeneous. 



It appears then that, according to the theory of undulations, 

 we ought to have precisely the phenomenon which Mr. Potter 

 has observed, supposing the light heterogeneous. It appears 

 also that a diminution of the heterogeneity will not alter 

 the place of the centre of the fringes ; its only effect being to 

 make a greater number of bars visible on both sides of the 

 centre ; except, indeed, the light were strictly homogeneous, 

 when the whole place of mixture of lights would be covered 

 with bars, of which no one could be called the centre of 

 fringes more than another. 



But this will not apply to Mr. Potter's experiment unless 

 we can show that the light used by him was heterogeneous. 

 I believe that any person who has tried experiments of this 

 kind sufficiently to know what brilliancy is necessary for 

 them, and how faint is light of any reasonable degree of ho- 

 mogeneity, will at once allow it to be probable. But I will 

 go further, and will assert that, on the face of the experiment 

 itself, it is certain that the light was heterogeneous. And the 

 reason is simple and unanswerable, that if the light had been 

 homogeneous there would have been no distinguishable centre 

 of fringes. And however small might be the heterogeneity, 

 the centre of fringes would occupy the same place as if 

 the light were (within certain limits) ever so heterogeneous. 

 The phenomenon observed by Mr. Potter is therefore, as far 

 as general explanation goes, completely in accordance with 

 the theory of undulations. 



The principle upon which I have determined the place of 

 the centre of the fringes, though it has been used by Sir John 

 Herschel in one experiment, has never (so far as I know) been 

 distinctly stated. It applies, however, in a great number of 

 instances, as well in the explanation of the phenomena of 

 polarization by interference, as in the interference of com- 

 mon light The following instructive experiment (which has 

 frequently been alluded to) shows very clearly to the eye the 

 application of the principle ; and I cite it the more readily, 

 because I infer from a passage in Mr. Potter's paper that 

 he may not be unwilling to try it. I shall describe it in the 

 form in which I have found it easiest in practice. 



