390 PROCEEDINGS OF THE SOCIETY. 



as they had just heard unless it could be seen in full and in print, l:)uthe 

 nnderstood that the artificial star employed for the purpose of the 

 demonstration had been formed by passing light through a small hole in 

 a diaphragm. The diameter of the ring was therefore not the image 

 of a mathematical point of light such as a star, the diffraction fringe 

 being formed about a small surface, the geometrical image of the hole, 

 and it looked to him as if the aperture of the hole described by Professor 

 Porter had an angular measurement of about 2h seconds of arc. In 

 order to reduce the value of this to that of theory, it would be necessary 

 to deduct from the observed ring the value of this geometrical image in 

 •order to get the reduced diameter. He hoped, therefore, that when the 

 paper was printed Professor Porter would give them the material from 

 which the angular value of the ring could be determined, because he did 

 not gather in listening to the paper that the figures put before them 

 comprised the necessary data. He was sure that when they could read 

 the paper in its complete form the Fellows would feel greatly indeljted 

 to Professor Porter for bringing this interesting subject before the 

 Society. 



Mr. Conrady said he had listened to the paper with great interest, 

 for he had been hoping that some reply would be made to the statement 

 of Mr. Nelson which had been referred to, since he rather thought Mr. 

 Nelson had got a little mixed up over the gi'eat variation in the 

 rings which took place as the telescope was racked out. He under- 

 stood that Mr. Nelson dealt with the results ol)tained with an 

 ordinary object - glass, whilst the theory implied that an absolutely 

 spherically correct object glass was used, and this was not yet produced 

 by anyone, as the best corrected object-glass always gave a slight amount 

 of spherical aberration, causing a slight distortion. The cause of a 

 distorted wave had been brought to a theory by Dr. Stoffer, of Jena. 

 He believed, therefore, that what happened in Mr. Nelson's case was 

 that he had used an objective with a slight spherical aberration. He 

 thought that Professor Porter, in arranging his experiment, had done 

 a wise thing, and one which was more likely to give greater accuracy. 

 He was sure that Professor Porter had given them a very interesting and 

 useful paper, and for his own part he would rather back his figures than 

 those which had been given to them before. 



Mr. Beck said that the point which struck him in Mr. Conrady's . 

 suggestion was that it was difficult to see how aberration or focusing 

 could reduce the size of the rings, though it was easy to see how it 

 might increase it. 



Professor Porter, in reply, said that he would take care to give in 

 his paper the full details from which they would be able to calculate the 

 angular aperture of the point of light, which was, as a matter of fact, 

 about 80 ft. distant. He might say that in arranging the experiment 

 this matter had not been overlooked, and that the pin-hole chosen was 

 sufficiently small to render negligible the error arising from its size. 

 He did not care to guess what happened to Mr. Nelson, but he felt sure 

 that Mr. Nelson, as well as himself, had used a diaphragm placed in 

 front of the telescope, and he had written to him about the subject 

 because he had thought there was a possible explanation to be made as 

 to the cause of the discrepancy ; this, however, he now thought was not 



