. PROCEEDINGS OF THE SOCIETY. 483 



I hold no brief for the Abbe theory. That too, to my mind, wants 

 further simplification and extension, so as to undeniably include, or at 

 least show, its bearings to the results arising from cones of light, from 

 self-luminous objects and isolated objects, as well as gratings. Work 

 has been already clone in this direction by Dr. G. J. Stoney, in his mono- 

 graph on Microscopic Vision already referred to.* 



That a number of deductions harmful to the progress of Microscopy 

 have been drawn from the Abbe diffraction theory, particularly the un- 

 certainty as to the correctness of the image and the desirability of using 

 narrow cones of light, seems undeniable in the face of the practical ex- 

 perience of to-day. But, whilst parting with the deductions which are 

 erroneous, we must be very careful not to throw away the valuable 

 fundamental principles. 



After all, the Abbe theory, on the basis of plane waves, like the other 

 theories on the spurious disc method, such as Lord Rayleigh's, is but a 

 mode of treating the subject. A subject can be treated in various ways, 

 and the one need not be rejected because we may use the other also. We 

 have an example of this in the deductions as to the resolving power of 

 the Microscope. Both the modes above named give approximately like 

 results, because both are dependent on the same essential factor for 

 resolving power, viz. the angular aperture of the objective. 



It so happens, that with most problems as to microscopic vision the 

 Abbe theory lends itself to them far better than other theories, because 

 it is the only one at present which takes into account what happens to 

 the light between the object and the objective, and this alone is a suf- 

 ficient reason why we are bound to retain it. 



May I say, in conclusion, that if, in this discussion, the remarks 

 made have been directed more to the points of difference between 

 Mr. Gordon's opinions and mine than to the points of agreement, I hope 

 it will not be held that I wish in the slightest to detract from the merits 

 of a paper which every one interested in microscopic vision must admit 

 to be one of the most valuable contributions to the literature on the 

 subject. 



Mr. C. Beck said that before the discussion was closed he should like 

 to say that he did not think it possible for any one discussing the paper 

 in opposition to Mr: Gordon, and following the experiments which he 

 had described, to dispute his contention that the effects observed were 

 produced by the diaphragm behind the objective. The proof that these 

 effects were entirely due to this was abundantly shown by the fact that 

 the moment any of the conditions were altered the experiments did not 

 succeed, and there was no reason why they should not succeed if the 

 Abbe theory were correct. 



Mr. Gordon contended that he was entitled to claim the support of 

 Prof. Thompson, notwithstanding the impression probably left on the 

 minds of those present by Prof. Thompson's speech, that he disagreed 

 with the reader of the paper almost entirely. It was true Prof. Thompson 

 had said he agreed with him in throwing over Naegeli and Schwendener's 

 explanations, but he also said that it was extremely wicked to throw 

 over the Abbe theory ; whereas it was clear from the quotation given at 

 the beginning of the paper that Prof. Abbe had himself thrown it over; 

 but in doing so he had promised to elaborate it further ; and as he had 



* Philosophical Magazine, October, November, and December, 1896. 



