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



is, that it may be seen both black and white, though with me 

 it has much oftener been the former, especially when the bars 

 have been well defined. I was so perplexed with this uncer- 

 tainty, that some time ago I wrote to Sir John Herschel, 

 though a stranger to him, to ask what mode of observing he 

 had used, and what were his results. I eventually, as may be 

 learned from the paper on Interference which I read at Oxford, 

 concluded it arose from the aberration in the focus of the lens 

 used to form an image of the sun ; and I accidentally, lately, 

 found, I think, the means of arriving at an unexceptionable 

 result. My two mirrors of speculum metal happened to be 

 so nearly parallel, that the images of the luminous point ap- 

 peared to the naked eye as only one; and when I looked 

 through the eye-glass ol f inch focus, I found that four bands 

 covered the whole field of view. I immediately endeavoured to 

 cause a shifting of the centre, which had been one of the causes 

 of perplexity ; but found from the great breadth of the bands 

 that this effect did not now perceptibly take place, and that 

 the central band was undoubtedly, unquestionably, a black one ; 

 as the colours were perfectly symmetrical on each side of it. 

 If this mode of trying the experiment is not the most unex- 

 ceptionable of any, I shall be glad if those who suppose the 

 central band to be always white will set me right, and show 

 in what mode we may try it, so as to settle so important a 

 point. 



Sir John Herschel has expressed in print his opinion, that if- 

 equal talents had been exerted on the corpuscular theory which 

 have been exerted on that of undulations, it is probable that the 

 phenomena of optics, supposed to be referrible only to the 

 latter, would have been found to be well accounted for on the 

 former. I have seen nothing in the course of my experiments, 

 nor met with any thing in the course of my reading, which 

 could lead me to dissent from this opinion, but rather to go 

 beyond it. 



The establishment of truth, alone, should be the object and 

 pride of all engaged in scientific researches ; and though it is 

 always more pleasant for the time to find new truths chime in 

 with old opinions, yet it is a matter of weakness to allow their 

 discordance to give us any lasting vexation, or to prevent us 

 from giving them openly and fairly to the world. I hope that 

 neither my opponents nor myself will ever want sufficient 

 courage to publish their researches, or to confess a change of 

 opinion when it overtakes us ; and that such change will not 

 cause in any a relaxation of zeal in the prosecution of scientific 

 inquiries. 



With respect to the solution of the question of prismatic 



