92 



the large solid axial cone had a greater resolving power in it 

 than was generally supj^osed. His experience showed him that 

 80,000 times the N.A. of the objective was the resolving limit 

 in inches with this kind of illumination, but from what Mr. 

 Merlin had said it was evident that a larger coefficient must be 

 employed. The little beads in the lines on the hoop of a 

 Pinnularia major were, so far as he knew, unresolvable by 

 oblique light, but with the f ths solid axial cone he had been able 

 to see them with the dry 4 mm. apochromat. Strange to say, 

 this same object had in 1895 been a kind of minimum visible 

 •or crucial test for an apo. J " of 1*43 N.A. ! 



It appeared, therefore, that the "minimum visible," the "crucial 

 test," the "scarcely resolvable detail" of one year became the 

 commonplace object at a subsequent period. This, Mr. Nelson 

 said, had been his frequent experience during the quarter of a 

 century he had been actively engaged in microscopical work. 



Dr. Spitta said that seeing things with a direct solid cone was 

 no doubt very much better and more to be relied upon than 

 seeing them by oblique light. By means of a further diagram 

 he showed that an object with large markings well seen by 

 •direct light appeared simply grey with oblique light. If the 

 lens employed was a fine one and the lines were very fine they 

 could be seen with oblique light, but if they were coarse they 

 were lost sight of by virtue of their largeness. 



Mr. J. M. Oftbrd said that reference had been made to Mr. 

 Merlin's eyesight. He could say that it was exceptionally good 

 with the telescope, for when Mr. Merlin spent an evening with 

 him and they were looking at the moon through his telescope, 

 he found Mr. Merlin certainly saw minute details on the lunar 

 surface which he himself had great difficulty in seeing even after 

 they had been pointed out. 



On the motion of the chairman hearty votes of thanks were 

 accorded to Mr. Merlin for his paper, and to Dr. Spitta for 

 reading and making his own valuable observations upon it. 



Mr. C. F. Rousselet read a paper " On the specific characters 

 ■of Aspkmchna intermedia,'' which he illustrated by diagrams and 

 by specimens exhibited under several microscopes. 



Mr. Western said he looked upon this as a very valuable 

 contribution to their knowledge of this genus, as there had 

 always been great confusion as to these Asplanchnas. He had 



