396 PROCEEDINGS OF THE SOCIETY. 



thoughts on microscopical theories. It seemed difficult to understand 

 at the present day why Abbe should have defended so persistently, and 

 with his characteristic vigour, the use of small illuminating pencils ; 

 possibly the imperfections of the earlier objectives had something to do 

 with it. There could be no doubt that, given objects and objectives 

 which would bear it, the wide and well-centred illuminating cone had 

 every advantage, for whilst a narrow oblique illuminating pencil would 

 realise the fullest resolving power of an objective, it did so in one direc- 

 tion only. One might see delicate structure in one direction, whilst far 

 coarser detail at right angles would be invisible. The wide axial cone, on 

 the other hand, gave nearly the full resolving power attainable with 

 oblique light, but gave it uniformly in all directions, so that all detail 

 accessible to any particular objective was seen simultaneously. Another 

 thought which occurred to him concerned theories in general. The con- 

 ditions prevailing in ordinary circumstances of microscopical (and other) 

 observations were far too complicated to admit of treatment even by 

 present-day mathematics, and the first thing to be done in framing any 

 theory consisted in eliminating the complications until the simplified 

 conditions admitted of mathematical treatment. The result would be a 

 theory which should be rigorously borne out by experiment, provided 

 that the underlying assumptions were realised, but which might seriously 

 disagree with the result of observations if the proper conditions were not 

 fulfilled. 



Mr. F. Shillington Scales was pleased to have heard the interesting 

 remarks of Mr. Conrady, and he thought that, after all, the actual 

 theory of the Microscope did not affect five-sixths of those who were 

 Fellows of the Society, and still less other workers ; what did affect them 

 was the question of how to get the best results out of their Microscopes, 

 and the important point was to get a true representation of the structure 

 under examination. He thought that no one who had done really 

 difficult work with the Microscope could have any doubt that Mr. 

 Nelson's method was the correct one, and modern microscopy owed him 

 a debt for his frequent enunciation of correct principles of working. 

 Except by the use of large axial cones, well corrected, and bearing a 

 definite ratio to the aperture of the objective used, it was impossible to 

 obtain the full advantage of the fine objectives which the skill of opticians 

 of the present day had given us, and Mr. Nelson had done more than 

 anyone else to call attention to this. He was glad to find that Mr. 

 Nelson laid stress upon the unreliability of bacteria as a test for objec- 

 tives ; anyone else who had had experience in testing objectives would be 

 able to indorse this view. 



The Chairman said they all knew Mr. Nelson well enough both by name 

 as well as by his numerous contributions to Microscopy, but those who 

 recollected his earlier writings would remember the severe attacks made 

 upon him by his hasty critics when he advanced his views upon critical 

 illumination. Well might he have told them to " wait and see," for they 

 are certainly accepted by every thinking microscopist in the present day. 

 They were glad to receive another paper touching upon this subject, and 

 he felt sure the Members present would desire it to be recorded they 

 were pleased once more to receive his communications, and to express the 



