Mr. Potter's Reply to Professors Airy and Hamilton. 277 



proper strength, gives a much purer light than the red glass 

 found in the windows of old churches, and which M. Fresnel 

 considered sufficiently homogeneous for experiments where 

 much greater delicacy was required. I also inclined the two 

 mirrors so much to each other as to render the bands suffi- 

 ciently narrow; and when I observed the bands, similar to a b 9 

 in the figure, come, by withdrawing the eye and eye-glass a 

 little distance from the prism, into a po- 

 sition similar to c a, 1 had no danger 

 of falling into Prof. Airy's error of sup- /^^i^^C 

 posing the change to arise only from 

 the shifting of the centre of the fringes, 

 whilst the bars themselves remained sta- 

 tionary. The dotted lines ef represent 

 the diffracted fringes, caused by the 

 edge of the lower mirror. 1 cannot 

 imagine how Prof. Airy should repre- 

 sent me as describing the appearances 



by a shifting only of the centre of the fringes ; — my description 

 states distinctly the bars themselves to move ; and Professor 

 Hamilton has evidently thus rightly read it. By operating in 

 the manner above described, and placing the prism so that its 

 edge appears to touch the bands formed directly in the air, 

 then looking with the eye-glass at a little distance from the 

 prism, another complete set of bars may be seen in the pris- 

 matic light. I have repeated this experiment frequently, and 

 believe Professor Powell and myself succeeded in so trying it 

 at his residence in Oxford in June last. 



The phenomenon of the shifting of the apparent centre of 

 fringes must be noticed in the common experiment, without a 

 prism, by every one who frequently tries it, as it occurs per- 

 petually, when, from looking directly, we change the position 

 of the eye, in the plane perpendicular to the direction of the 

 fringes, and look obliquely through the lens. 



From what I have just said, it will be seen that I consider 

 it no compliment that the Plumian Professor should think me 

 ready to advance the minute effect of the shifting of the centre 

 of fringes with light slightly heterogeneous, as a strong and 

 solid argument against the undulatory theory. I must also 

 respectfully inform him, that I believe the probability of my 

 becoming an undulationist becomes daily less and less; as, 

 from the time of my having merely an opinion upon the ge- 

 neral theory, from having read Dr. Young's Bakerian Lecture, 

 I am now gradually come to see many serious and weighty 

 objections against it, of which several have the greater influ- 

 ence with me from having arisen in my own experimental in- 



i hoihjinunwno 



