1898.] MICROSCOPICAL JOURNAL 117 



necessity depends on the nature of the object, and does 

 not exist in all cases. 



19. There is a very obvious and simple, yet decisive 

 test as to the correctness of this view. According to the 

 Abbe or spectrum theory, the amount of cone or hetero- 

 geneous light which can be' used will depend upon the 

 minuteness of the structure alone. According to the 

 view here maintained (which recognizes the Airy theory 

 as also concerned in the image) the density or contrast 

 of the structure is the chief factor in this question. All 

 experience proves that the latter is the case. 



It ouly remains to show how directly the questions 

 here discussed affect practical microscopy and the work 

 of the microscope optician, and also determine the pros- 

 pect of further advances in our powers of microscopical 

 research. 



20. The Abbe or ''spectrum" theory has in its time, 

 confessedly, led to enormous improvements in objec- 

 tives. Owing to that specialization and ignorance of 

 what physicists had done, there was amongst microscop- 

 ists no undei"standing of the direct function of aperture 

 in resolution ; and so the Abbe theory was for years 

 written about, and advanced as " the first explanation 

 ever given." It thus produced a vivid consciousness of 

 that function which was entirely new, to which we owe 

 our present immersion and other high aperture lenses. 

 But it is as easy to show that, this work being done, its 

 undue prepouderence and acceptance as the 07ily theory, 

 especially on the Continent, is now causing distinct pre- 

 judical results, owing chiefly to its connexion in practice 

 with a narrow pencil or cone. Abbe himself throughout 

 insisted upon the narrow pencil. Dr. Van Heurck does 

 the same ; Dr. Peragallo writes that a cone of more than 

 0-50 N A. is of no use ; and Dr. Dallinger, and authori- 

 ties like him, who in a general way accept the Abbe 

 theory as the " theory," but know from their own exper- 



