Mr. Potter on Diffraction in the Shadow of a Disc. 151 



seen, I could not avoid looking to the position of the writer, 

 as well as to my own position, and forming my decision as to 

 the course which was proper on my part. I have only to add, 

 that nothing could be further from my intention than to give 

 a personal character to this controversy, and that I trust no 

 expression has escaped me which will bear such an interpre- 

 tation. I am, Gentlemen, 



your obedient Servant, 

 Royal Observatory, Greenwich, G. B. Airy. 



July 10, 1841. 



XXV. On the Phenomena of Diffraction in the Centre of the 

 Shadow of a Circular Disc, placed before a luminous Point, 

 as exhibited by Experiment. By Richard Potter, Esq., 

 M.A.* 

 'T'HE results of the undulatory theory, to which I called 

 ■*■ attention in the Number of the Magazine for October 

 lastf, being opposed to the received opinions upon those points, 

 it became desirable to undertake an experimental examination 

 of the most decisive case ; namely, the nature of the phas- 

 nomena in the centre of the shadow of a circular disc when 

 placed directly before a luminous point. According to that 

 theory the point in the centre of the shadow ought to be as 

 bright as if the disc were not interposed ; and this being ac- 

 knowledged on all sides, we have an admitted and clear test 

 of the theory, as long as it is allowed to be a theory which 

 may be tested by experiment. 



The labour which the Astronomer Royal has devoted to 

 deducing the numerical magnitudes, given in his paper in the 

 January Number^:, has also rendered this experiment more 

 interesting, and I may anticipate the result at which I have 

 arrived, by stating that the undulatory theory in this, as in 

 so many other cases, fails entirely. 



Before relating the experiments, I must notice some of 

 Mr. Airy's. statements as to the visibility of luminous points, 

 and the relative brightness which renders neighbouring spaces 

 which are differently illuminated distinguishable from each 

 other by the eye; because an incorrect assumption on these 

 points would affect the whole discussion. Tocommence with the 

 latter, he says, " Thus there will be at the centre a bright 

 spot of double the general intensity, surrounded by rings 

 brighter than the great expanse of light ; but the excess of 

 intensity in the rings, even the first, is so small, that it pro- 

 bably could not be seen." Now the intensity of the minima 



* Communicated by the Author. 



t Third Series, vol. xvii. p. 243. J vol. xviii. p. 1. 



