PKOCEEDINGS OF THE SOCIETY. 259 



The reason why Mr. Gordon obtained an apparently opposite result 

 in an experiment which he showed on tw'o occasions is probably that his 

 experiment did not comply with the conditions usually prevailing in the 

 use of the Microscope, and that it also did not enable one to separate 

 the effect of a change of focus from the disturbing effect of a simul- 

 taneous change in the angle of the illuminating cone. Such mixed 

 effects are quite inadmissible as evidence in scientific investigations. 

 In Dr. Stoney's experiment this mixing of different effects and de- 

 parture from normal conditions is carefully avoided by using the 

 ordinary equipment of the standard Microscope, thus enabling one to 

 study the effects of change of focus independently from changes of 

 angle of cone and changes of illuminated area. 



The same reproach of being behind the times applies to Mr. Gordon's 

 last statement : " The Abbe theory takes account of ' diffraction ' 

 w'herever it occurs." It first notes that which takes place in the 

 object ; it then closely watches the passage of all the light through the 

 objective, and notes whether all of it is admitted or whether part of it is 

 cut off, considering only that which is admitted. But whatever may 

 be the condition of the light entering an optical instrument, the only 

 function of the limiting aperture is to determine the area through 

 which light can enter, and as the Abbe theory necessarily considers this 

 limiting effect of the aperture, it takes " apertural diffraction " into full 

 account. 



The idea that diffraction took place only at the sharp edge of the 

 limiting aperture is a hopelessly antiquated one. The final image is 

 always due to the interference of all the light which enters into an 

 optical instrument, no matter whether that light has come uninter- 

 ruptedly from a self-luminous object, or whether a greater or lesser part 

 of it is due to a preliminary " diffraction " in an artificially-Hghted 

 object. 



Mr. Rheinberg said that those interested in the subject knew that 

 Mr. Gordon was one of the few who regarded Abbe's epoch-making 

 work on the theory of image-formation in the Microscope as incorrect. 



So far as the objections mentioned that evening were concerned, he 

 thought these had all been brought forward by Mr. Gordon years ago, 

 when they were fully dealt with in discussion, and to his mind, amply 

 disproved. He agreed with Mr. Conrady that the matter turned on the 

 question of diffraction by the object, which Mr. Gordon thought he had 

 disposed of when the objective was filled with light. But this was 

 fallacious — though masked, it was present and effective all the time. 



Regarding the question of " apertural " diffraction, it was quite a 

 mistake to suppose that this had not been taken into account by Abbe — 

 he could say this from discussions on the subject with those who were 

 intimately associated with the latter — and it might also, he thought, be 

 found in Abbe's papers. As a rule, however, the diffraction by the 

 object played a role so much more important than that due to the 

 diffraction by the aperture of the objective, and it became unnecessary 

 to lay much stress on the latter. 



Mr. Gordon said that he would not follow Messrs. Conrady and 

 Rheinberg on the personal points which they had raised, since that 



