PEOCEEDIXGS OF THE SOCIETY. 767 



the other, but this was not really the ultimate limit, which was still closer. 

 What he and ]\Ir. Everitt had questioned was that Mr. Xelson had o;iven 

 the measurement of the first diffraction ring, as he seemed to claim to 

 do. They did not deny that Mr. Xelson had gone considerably beyond 

 the conventional limit, and theory showed that a closer approach than the 

 conventional limit was possible. Professor Porter suggested that it might 

 be better to take as the true limit the closeness for which the midpoint 

 of the double image just failed to show a diminution in intensity. 



Mr. J. ^Y. Gordon said the question was one of extraordinary interest, 

 and he hoped at some future time to have the opportunity of making a 

 contribution to the discussion of the subject which at that late hour of 

 the evening woiild not be convenient. He would only suggest a doubt 

 as to whether the calculations to which Professor Porter had referred 

 applied at all in the empuical focal plane, as he thought it quite possible 

 that the empirical focal plane lay below the theoretical focal plane for 

 aplanatic rays. He should like to elaborate that suggestion. Another 

 observation which he would like to make was that what Professor Porter 

 had been talking about was the image formed by a luminous point, such 

 as occurs only in practice when a star is observed. These autipoint 

 curves do not apply in the case of a small area such as constitutes the 

 minute detail seen in a Microscope. In discussing those, we need to 

 take into account not only the bright diffraction fringe which lies upon 

 the dark field, but also the dark difl^raction fringe which borders the 

 bright field, and which has an equally important bearing upon the 

 problem of resolving power. 



The President, in moving a vote of thanks to Mr. Xelson and those 

 who had taken part in the discussion, hoped that Mr. Gordon would on 

 some future occasion exhibit some more of his beautiful photographs, and 

 also that he would give them the benefit of his views of the subject. 



Mr. MoflFatt read a paper " On Light Filters for Photo-micrography," 

 exhibiting in illustration of the results obtained some very fine prints of 

 Trypanosoma, bacilli, etc.. taken in the manner described. 



The thanks of the Meeting were voted to Mr. Moffat for his paper. 



Mr. E. F. Law gave a demonstration of the use of colour photography 

 in metallurgy, exhibiting a number of very lieautiful slides showing the 

 brilliant colours produced on the polished surfaces of alloys by the 

 varying degrees of oxidation of the different constituents. 



The President said they were greatly obliged to Mr. Law for this very 

 interesting exhibition. Many of the shdes shown were very beautiful. 

 The thanks of the Society were voted to him unanimously. 



Dr. Hebb said they had received a letter from Mr. Macphail, accom- 

 panied by a packet of material consisting of Radiolarians for distribution 

 to such Fellows of the Society as wished to have some ; samples could be 

 obtained on application to Mr. Parsons. 



On the motion of the President, the thanks of the Society were voted 

 to Messrs. Beck for the loan of the Microscopes for illustrating the 

 subject of Mr. Gordon's paper. 



